This question came from a student in the How to Write a Children’s Book course on Skillshare.
The level of complexity is entirely subjective.
Everyone learns and experiences reading differently. I would encourage you, the writer, to try writing for your younger self.
You could model your writing after your favorite children’s books from when you were a child.
You could also pick a child in your life to write to, molding your words specifically for them.
It’s all up to you, my friend.