I’ve been writing a book lately and it’s about how this guy starts a family and he has a daughter, but the story starts before she’s born. How do I segue her POV into the story without it sounding forced and weird?
Does an event signal this segue? A change of setting? I would say that the main concern would be to give us a reason why we are now seeing the story from her point of view and not from his. Think of other stories that use multiple POV. A Song of Ice and Fire, for example, uses multiple POV's because we need to know what is happening at different points of the seven kingdoms. In The Lord of the Rings we see the division of the Fellowship and that leads us to follow two groups. A closer example could be Star Wars. We start the prequels with Anakin as our lead but after his fall we move to his children, Luke and Leia, as our main POV, with Vader taking a secondary role.
Basically I think the main question is why are we going to need to now see the story from that POV. If you have that then it should be no problem and wont be weird or forced.
In a more practical and technical answer, you could use the names as chapter tittles or divide the book in sections where that signal this shift (name them after places, seasons, time, or just words that associate with the character who we are following).
1st-person POV is all about character.
The POV character isn't just relating a bunch of events they "see" in front of them like a GoPro camera mounted on their hat. They are telling their version of what happened, what was experienced from their perspective. They focus on the things that are important to themselves. Other things, which are important to the reader and the other characters, might only be mentioned casually if at all.
Once you jump to the new character's POV, the reader will naturally be looking for familiar things they already know through the other character, except now the familiar will be seen from a whole new perspective. It doesn't matter what kind of story (spy novel, romance, family secrets) the reader will be looking for common elements described from both POV to act as "signposts" for how these characters overlap.
In your story the father and child share the same house so almost everything in the child's world is included in the father's. Almost everything will be a "signpost" so what you can do, at least at first, is see these familiar objects in radically new ways. That comfort toy that the father uses to keep her quiet, has a very different significance to the daughter.
As she gets a little older her POV will start to introduce play and imagination to her descriptions. She can still communicate factoids to the reader, but mostly she will filter everything through the screwball brain of a kid. She will be an unreliable narrator but in very obvious ways which communicate her personality (the stuffed animals are always afraid but she isn't, or two dolls are best friends but one isn't very nice). As she grows up, her POV will become more aware of the real world but she will always fail to understand some things. She is never omniscient, and her imagination is still there but she begins to learn what to say and not to say long before she understands why she shouldn't say certain things.
Meanwhile the father, who up to a certain point in the story has been the only voice, is automatically granted authority through his POV. Everything he says is accepted by the reader as factual unless proven otherwise. There is no reason to suspect he is also an unreliable narrator (everyone is) until the second POV appears. It's not necessarily that he is "wrong", but he isn't concerned with the fantasies in his daughter's head so much as he just needs her to stop crying and go back to bed.
As we learn he is not omniscient, we start to see his limitations as a human being. I don't know how long your story follows these two, but the day a child realizes their parent has limits is the first day that child starts to grow up and become independent. You have an opportunity to parallel the daughter's growing understanding of the world with the reader's growing understanding of the father character.
Don't worry whether readers will follow the POV shift, they are not stupid. You won't need to spell this out for them, the change in POV from an adult male to a little girl will be obvious the first time it happens. You don't need to warn people with chapter titles or elaborate printing tricks. Let the character tell what is important to them in their own voice.
Develop each character's Theory of Mind, especially their opinions of what the other is thinking because these are specifically things that the reader will be allowed to know when one character got it wrong. 1st-person POV can be rich in character personality, but it becomes even richer when we can compare the same events through two different POV. Set yourself this goal of telling the same story from two different characters from the start, decide early on what each character doesn't know – the things that will be omitted from their POV. When the reader has to do some brain work to reconcile conflicting accounts which are stated as fact, the characters begin to feel multi-dimensional as if they exist outside of the words in the story.