Well, that is difficult. You want to make an abridged encyclopedia of everything in a single volume, yet entertaining and interesting for a kid...math, science, history (like the song goes), philisophy, human development, society..
Check out your competition, which would be any childrens Encyclopedia on Amazon, or the ancient mythologies which explain everything about everything important that the authors knew about. If you're trying to get a body of essential knowledge into the hands of the reader read homeschooling books from the library (Susan Wise Bauer, Lisa Rivero). They even say the Tarot Deck was written by Gypsies to cover all the bases on every aspect of the human condition. I have it on my bucket list to write a Tarot deck for preschoolers, and bind it into a pictures only encyclopedia of the world so that mothers (in general, not gifted-specific) can educate and converse with pre-literate children the journey of the human condition.
I think you have a great idea and that is to write it to your younger self or to your own (future (?) kids). I personally want to start a homemade children's dress company. I've made one dress so far, it's sitting in a store waiting to sell. I tried to imagine what image or style I wanted for my dress company. What made it easier was to realize that if the dress does not sell then my daughter will grow into it and wear it. I picked fabrics to make a look that would ne darling on my daughter. In other words, if you want people to like you, then become something that people like. Your book needs style at least as much as substance.
