My story is about two children and one of them dies in the first chapter.
I give a four page backstory of how his parents came to the town they are in now, it is unnecessary but very interesting. Should I just remove it?
I mostly added it because the chapter was too short and I wanted to expand on how the dead one came to the town but if I never really talk about him again is it just unnecessary to have all that back story about his parents?
I think that you already know you have to cut it. I know you are emotionaly attached to it but, if it doesn't move the plot forward and if it doesn't help establish character,... well, as you said, it's useless.
Adding text just for the sake of adding it is not a good idea. But before removing it, ask yourself why you think it's interesting.
Is it because you care about your character and want to know more about them? If it is, just remove it. At the begining of a story, no one cares (yet) about a character and their backstory. All the reader wants is to find out what is the story gonna be about and what the character will do.
Is it because it shows that the character has this and that characteristic? Then keep it, it add depth to the character.
Does it give information that we are going to need later? Then you can keep it but make sure that there are other interesting things in the chapter (and not just four pages of info dump, info dump is boring).
After answering these questions, and because the answer will probably be a mix of all these things, you have the following options:
Remove everything from the story (but keep the text for you so you can use a slice of it later).
Reduce the passage to keep only what is "necessary".
Don't do anything and wait for when you have finished the story to take a decision.
If you decide to keep everything, make sure to re-think your decision at the end of the story to be sure you did the right thing.
Have you considered revealing the backstory piecemeal? If the other child survives whatever kills the first, maybe the first child will be regularly fleshed out by the survivor's thoughts, feelings, and memories of her.
Instead of dumping four pages on a reader out of nowhere, consider organically weaving whatever backstory is necessary into the survivor's feelings on the matter. Surely they'd think about their dead childhood friend/sibling regularly enough, especially in hard times.
This is known as a posthumous character, and it's quite a common device. It's where a character is only fully realised via how they affected others with their deaths.
Chapters have been as short as a single sentence; Stephen King is famous for a single sentence chapter. Other famous authors have written chapters of a single page.
In general, a chapter break occurs when the story changes POV, or there has been a space jump or time jump AND a change of cast. We change chapters for the same reason a stage play closes the curtains and re-opens them on different scene.
If there is JUST a time and/or space jump; you don't have to change chapters; you can cover the transition in a paragraph. Bob and Charlie are in Seattle, we use a paragraph cover the uninteresting travel part of the story, then we are in Miami. For example:
Eight hours later, they landed in Miami. They took a taxi from the airport to the Greenpoint Hotel, and found their room on the fifth floor.
"You take the left bed," Charlie said. "I can't sleep with somebody to my right."
"Yeah sure," Bob said. "We going out for dinner? You want to order something in? They got this list of places."
He held up a colorful brochure of restaurants that delivered to the hotel.
"Nah. We'll ask the front desk, find us a Chinese place we can walk to. After all that sitting I got to pump some blood and get my legs right again."
Done! The transition is that two sentence paragraph; The reader is beside them in Miami, we sneak in something about the uncomfortable nature of the ride, Bob and Charlie are firmly in the Greenpoint in Miami. (Which may or may not exist, I just made up this little story for this example).
But YOUR chapter probably makes sense as you have it, make it six pages without the back story.
That said, it is likely a different mistake to kill the kid so soon. A story generally opens on "the normal world" for the MC; how life is for them before it changes. The change here (a sibling dying) should NOT occur in the first chapter, it should occur in the 2nd or 3rd.
So I think you are probably rushing your story. It is The Normal World that introduces our MC, perhaps the family and the doomed boy. Killing somebody the reader does not know, to affect somebody the reader does not know, does not affect or engage the reader much at all. You are giving them all strangers, saying some stranger died, and expecting that to have some emotional impact on the reader.
It will not. Readers must be introduced to the character first, and THAT is what Chapter 1 is for. Whomever your MC (main character) is, tell some of this backstory in Chapter 1 through their eyes, how they felt, where they left, and in particular interactions the MC has with their brother, the doomed boy. Make the doomed boy sympathetic: Fun to play with, innocent, whatever, a typical kid having reckless fun, looking to the future, excited to be in a new place, going to a new school, meeting new neighbors and friends already, until you put him in the blender. THEN his death means something, to the MC and his parents that loved him.
Typically in a story, the "Inciting Event" that the story is about will occur 5% to 10% of the way through the story. (Read up on the Three Act Structure). If you are in a hurry, you still need about 5% of "Normal World" before you do something drastic. In a 300 page novel, with 250 words per page, that is 15 pages. I'd suggest the following structure:
Chapter 1, The Normal World. For you, the family in transition, getting settled, particularly for the MC and the doomed boy. May be short; 2% of the whole story.
Chapter 2, time jump to a more settled Normal World, perhaps months later, kids in school with friends. Pick an event: A birthday, fourth of July, last day of school, whatever. At the end of Chapter 2, kill the boy; that is the final scene of this chapter and the "inciting incident". This can be 3% or so of the story.
Chapter 3, time jump to "the new world", perhaps leaving the funeral, or days after the funeral, with the family still grieving; that would be the natural aftermath.
The recommendation to remove something that's good in its own right, but doesn't belong in the broader context of a work, is usually phrased as "kill your darlings". While this is now a literary journal's name, it started when Arthur Quiller-Couch used a slightly different phrase in a 1916 lecture:
Style, for example, is not – can never be – extraneous Ornament. You remember, may be, the Persian lover whom I quoted to you out of Newman: how to convey his passion he sought a professional letter-writer and purchased a vocabulary charged with ornament, wherewith to attract the fair one as with a basket of jewels. Well, in this extraneous, professional, purchased ornamentation, you have something which Style is not: and if you here require a practical rule of me, I will present you with this: ‘Whenever you feel an impulse to perpetrate a piece of exceptionally fine writing, obey it – wholeheartedly – and delete it before sending your manuscript to press. Murder your darlings.'
In that original context it's clear he was worried mainly about highfalutin vocabulary... you know, like highfalutin. Today, the concern is more with scenes. In either case, it's worth archiving your darlings so you can rework some of them later. One reason I use a program called LyX to write is that I can hide anything in a note, so it doesn't persist in the output but I never have to delete it. Chapters' worth have been preserved for future reference that way. But yes, if a scene isn't necessary, lose it. If the information in the scene is necessary, find a way to work its introduction into something truly necessary, be it just when it's needed or sooner.
I would tend to say that if it is truly interesting, it is probably necessary. You may need to work through figuring out why you wrote that, but until you complete the entire novel and sit down and read through it, I would not just cut something you find interesting. There is an underlying reason we find things fascinating, which may have to do with the story in a way that is not totally obvious. If it is strong, I would hold off until I saw the complete form of the story emerge, then decide. It is entirely possible to have sub stories which seem to be rabbit trails, which actually do contribute to the overall narrative and provide powerful emotional backing to the main plot.
Remember this: you are writing a story to entertain. Entertainment is interesting. Interesting pieces of a story are fulfilling the fundamental purpose of the work, because if the writer finds them so, the reader probably will as well.
it is unnecessary but very interesting.
For a story reader, unnecessary and interesting are exclusive. Either make it necessary and interesting or remove it.
Orwell's third rule to effective writing applies to passages:
If it is possible to cut a word out, always cut it out.
