My 7 year old is writing a book

So my 7yo is super creative and wants to write a book. He wants to sell it at his bonus mom’s booth she does at fairs.

I want to do everthing possible to make this come true for him and help him as much as possible. But I really don’t know where to start.

If anyone has any suggestions on creating children’s books even basic tips to help me help him I would very much appreciate it.