Make it a rule never to give a child a book you would not read yourself.
A children's story that can only be enjoyed by children is not a good children's story in the slightest.
You have to write the book that wants to be written. And if the book will be too difficult for grown-ups, then you write it for children.
If you want your children to be intelligent, read them fairy tales. If you want them to be more intelligent, read them more fairy tales.