I poked through the files for a while and eventually threw in the towel. That's a lot to absorb. If there was a "Go to declaration" feature for variables/functions/labels it would make things so much easier to digest. Full blown programming languages do that and it helps a ton.
A new child would be potentially be a simple matter with the right data structures. Essentially you have a child class, and each character has a dictionary of children.
class child(): [etc]
class character(): [etc, children = {}]
character.children["CHILD_NAME"] = child("CHILD_NAME", stuff, ..., etc)
You could then randomly generate children based on the character models. All of the data would be stored in the character.children attribute/property. And then stored locally however RenPy handles all of that.
You could even take it a step further and put age into character. Then the children are actually just character objects but based around the age value. character.children["CHILD_NAME"] = character("CHILD_NAME", AGE, stuff, ..., etc)
Unfortunately, all of this increases the complexity of your code. At a certain point the logic management becomes a pain. If age == young and is Poppy_Child and not Mitsuki.pregnant and etc etc etc. The conditional flow of your program can get overwhelming. Not to mention changing your data layout during a release is bound to create bugs
All big issues for game development. Thankfully RenPy keeps things mildly manageable. It's when you go wanting to add random objects like kids into the mix that you start poking into the heavy dev-end stuff.
I could toss you my discord or something if you ever wanted help or to talk deeper on possibilities. Automation through code data structures is typically what I do in my professional day-to-day. I'm more than glad to share the knowledge.