There are some people who have read some of my book so far and they think it is childish and then some say it's somewhat morbid.
What are your views on what each of those terms means?
It could be any number of things given that we've seen no excerpt (although sharing one may not be advisable, since advice specific to your case wouldn't help this thread's future users). I'll mention some possibilities; other answers will probably add some more. Some of my examples are things writers find more appropriate when writing for children; others are bad writing people may associate with stories aimed at children, if for these the publication standards are more lax.
The emotional lives of children, adolescents, and adults are very different. This sometimes lead adults to dismiss the emotions of children and adolescents as trivial or inconsequential, which is unfair. If anything, the emotions of children and adolescents are more deeply felt than those of adults.
With adulthood comes emotional maturity, which generally involves a dampening of emotions. One comes to recognize one's emotional reactions and learns to regulate them. You know the highs and the lows are going to pass, and so the highs are a little less high and the lows are a little less low, all of which is a good thing, because it means you are less likely to be undone by your emotions. You are able to carry on with your everyday activities even when it the grip of high or low emotions, which makes it much easier for you to show up for work on a sunny day and not throw yourself off a bridge when you are disappointed in love.
Childishness in literature is mostly evident as the lack of the emotional regulation that characterizes adulthood. One of the things you will notice about almost all children's literature written by adults is that it display an adult level of emotional control, both it is own tone, and in the actions and reactions of its characters. I think this is the natural reaction of adults to model emotional regulation to children, combined with the natural emotional regulation that adults have learned and more or less assume as the human norm.
Morbidity is also a lack of emotional regulation. It is an inappropriate dwelling on dark thoughts. (Inappropriate here really means that it is inconsistent with the emotional regulation that an adult learns to use to keep themselves functioning more or less cheerfully.) Dealing with dark subjects is not in itself a sign of morbidity. There is lots of adult work that deals with dark subjects. Morbidity is a lack of emotional regulation when dealing with those dark subjects.
Learning emotional regulation is just part of growing up. Certain life event, such as a first job, moving away from home, becoming financially responsible for your self, getting married, and becoming a parent, all force a greater degree of emotional regulation on you.
It is important to note that the emotions themselves don't change. It is the ability to regulate them that changes. Some people never learn it though. For most of us, it just comes with time and experience.
It really isn't a writing technique, so it can't be fixed by writing methods. As your emotional regulation improves in life, it will be reflected in your writing.
You are probably suffering from a case of Grimderp - when your dark parts go too dark and it stops being dark and becomes just silly. It is when a work of fiction goes so dark that it wrap around in the scale and becomes somewhat ridiculous for the reader.
Writing dark, emotionally deep stories is hard. It's extremely easy to get pumped up when adding details to a murder scene or a tragic moment, go over the top, and then lose the suspension of disbelief. My best example for this case is The Dark Tower, from Stephen King. While the world building of those books is extremely good, the overall story is painful to read if you aren't really, REALLY into King's work. The story of Roland and his quest for the Dark Tower is childish and morbid. But, why? What makes a work feel that way?
It mostly comes down to exaggerations. Suspension of disbelief gets more delicate the older you are - it's easier to break. Something that seems incredibly awesome for a fourteen year old may make my grandpa raise his eyebrows in confusion, wondering what the hell did I smoke and where he can get some for himself. Superpowers that destroy planets, demigods stuck inside orphans or super-perfect, hyper dark kids are the type of things that make very hard for a more serious reader to enjoy your work.
Think about Twilight. Those books are full of exaggerations, mostly around the protagonists. The things that happen, that unexplained "specialness" that a incredibly bland girl has, the one-dimensional aspect of the characters - it created a following, yes. But among what type people? How many of twilight readers didn't look back after a few years and thought "what was I thinking?" when looking back at the fan fiction they made for themselves?
The same goes for a few other works in other media. Naruto is a good example - while it may have been a blast to watch it with kid eyes, try watching it again a few years later. Then go to the mirror and try to explain to yourself why the hell you liked Sasuke back then.
Having a book rated "childish" or "too morbid" is probably a good signal that your tone is too over the top.
I'll answer this by providing a general commentary to the text specifically provided by OP, as I've just opened and read a couple of sentences of the Secrets Books. Bear with me, those were chosen at random, by selecting a random chapter and reading the sentence that caught my eye.
C2: "Walking through Kansas, Ryu complained to Haku. “It’s hooooot, when are we going hooooome?”"
C11: "“HAKU?!?! How are you talking to me? Where are you? Are you safe?” Ryu frantically spewed out these words. Haku’s silky voice laughed and sighed. "
C14: "“OHH I MISSED YOU SO MUCH!!!” Sumiko said while squeezing the boy in her chest."
It seems https://writers.stackexchange.com/a/28412/25183 actually hit the nail here without even reading or seeing the text itself. The sentence structure/grammar, vocabulary & themes all bring a young (aged 12-15) reader to my mind:
using mostly simple words with simple semantics (see below too).
Some more, also chosen rather randomly by skimming the text:
C7: "The royal family was pure evil, the Queen especially. The rumors about her are that she killed the King so that she could have full control of the kingdom."
C15: "In 2040, at the beginning of Earth’s colonization period, a new galaxy was stumbled upon and colonized."
C17: "As Ryu looked around him, darkness shrouded his vision, making his stress levels rise exponentially."
Finally, some (again, quite random) chapter titles are:
Titles like that (and/or a title of a book like "Secrets Book") are very common in low-quality literature, so using them is quite risky by itself, if you want to write adult prose.
Neither of that is wrong or bad by itself; it's just that using any of that stylistic options brings you a bit closer to having your writing considered "childish" (at best) by any more seasoned reader. You'd have to have a really epic & great story with strong protagonists & antagonists, a really immersive and innovative universe/world design, a great humor, and probably a couple of other "brain attractors" (see "Discworld" or, arguably, "Harry Potter" etc.) - to probably keep anyone who read through (e.g.) Tolkien, Le Guin, Lem and Dick from throwing a book with those "childishness" into the fireplace without even reading into it.
In total, all that "childishness" strongly suggest one of three:
I'm not betting on any of it, since I'm not reading through 93 pages of random prose that probably falls into either of the above categories. I can only say this: overusing anything leads to either an intended or unintended parody; if you have to do it, be sure you're doing it right - otherwise I'm not pay[ing] brisk money for this crap.
The thin violet glow was ice-cold against the rust-colored mountains. The Bryllis shrank to half an inch long and I worked fast stepping on them with the poltex. But it wasn’t enough. The sudden brightness swung me round and the Fourth Moon had already risen. I had exactly four seconds to hot up the disintegrator and Google had told me it wasn’t enough.'
(a fragment of SF parody courtesy of the Raymond Chandler)
One possibility not yet explored is subject matter and genre. Outside of the bubble, the majority of the public consider the fantasy to be 'childish'. LOTR and Harry Potter are essentially teen-fiction novels. DR WHO is a teen TV program.
I say 'the bubble' because on writer's sites the majority of members write fantasy. This bears no correlation to number of readers on the planet.
I will go ahead and attempt to answer this anyways. Childish writing usually refers to the fact that the word choice and sentence structure is basic as if it were a child. In the link provided, they go over the various sentence structures as well as provide examples of differences.
As far as being too morbid... that's just a matter of opinion really as well as the target audience. This is why I asked you in the comment who the book was designed for. Now, I should also add that you want to have a variety of tones and moods to the story. If the whole story was written from the viewpoint of Eeyore, one may find the story morbid and depressing/boring to read. If the MC of your story is of morbid nature, that is great! Keep in mind though that you need to balance everything out. Most commonly, you find in stories/movies after a heavy scene something more lighter, usually a laugh line or 2. Keeping it heavy the whole time will eliminate many readers as it would be niche at best.
I +1 to several of the above, and it is good advice; and I admit I haven't taken the time to read your work so far.
I'd add this observation: Writing will appear childish when the main characters (both heroes and villains) are made overly simplistic, ignorant, naive, or incapable or very limited in their reasoning or strategic thought.
Beginning writers will often over-explain things in exposition (details of their world, their characters emotions or motivations) or have their characters over-explain these things in dialogue.
But readers don't like to be treated as if they are five years old.
Along the same lines, young writers are often uncomfortable writing about adult romance or sex. Which is fine, stories can be written without that, but the writer should not aim to avoid it by writing characters that are NOT five years old with a five year old's understanding of romance and sexual attraction and activity. In other words you don't have to make adult themes central to your plot or description, but you shouldn't pretend they don't exist, or that any of your characters older than 13 (or past puberty) do not have any inkling about it.
This again is not treating your readers like five year old's, they expect characters that are physically adult to have the knowledge, motivations, and feelings of adults. A young teen can fake that, but don't do the opposite and have your characters so naive they don't know anything at all (unless they are all prepubescent children). You can fake it several ways. By "polite indirection" (e.g. 'At fifteen she thought she was in love, and became pregnant.') or allusion ('She was cheating on her husband, with his business partner.' or 'She was having an affair.'), or you can get yourself some indirect experience: Read other authors and see how they describe adult romance and sexual situations.
I focus on the sexual aspects not to be a pervert, but because adult romance, love and sex are common major ingredients of plots. They are also especially an obsession for those in the age bracket of 15 to 24: Somewhere in there most begin their own sexual lives; and both before and after it think about it often! Without any reference to it at all, a story may appear to them either written by a child or intended for children.
Isaac Asimov avoided sex in his stories, but remember he also said (I paraphrase) 'It is difficult to imagine a man risking his life in battles against overwhelming forces to save a woman because she is such an interesting conversationalist.' In other words, the love interest was present all along, so was the sexual interest, but the latter was not detailed. That does not mean it was ignored; behaviors and decisions and emotions must still be consistent with it driving the characters.
That does not apply if your characters and POV is nearly all children, like the Harry Potter stories: At 11, sex and romance are pretty far from the minds of the heroes. Authors completely averse to sexuality can similarly write with children heroes and avoid the topic completely. (Even for teachers, parents and other adults in the story; caregivers may be circumspect in their behavior and language around children and a prepubescent child's POV doesn't need a sexual imagination or suspicions).
Naiveté extends to other areas, as well; some of which can be researched online, others you may need the indirect experience of reading. Examples are the law, running a small business, being a politician, being a doctor, being a star of some sort (singer, artist, sports, musician, actor, writer), being wealthy, being a killer, being a criminal, being a soldier or Kung Fu expert.
My own stories include killers, but I have nothing remotely close to direct experience. Mine is indirect, I went back to authors I thought wrote convincing killers and studied how they presented them. I suspect they did not have any direct experience either, but I figured if they convinced me as a reader, then by emulating them, I could convince readers too.
So I encourage you to apply that lesson to writing about anything else with which you feel you have insufficient experience.