Children are like sponges and absorb information very quickly. Accordingly, you need to know what specific age you're writing for and how much they're able to understand; even a difference of one year can change their overall perceptions. Books for younger children should involve short sentences, simple words and dilemmas which don't cause undue distress (getting caught in the rain, for instance, instead of fighting a monster). Older children likely want more excitement in their stories and can handle more complex uses of grammar. Read up on child psychology and teaching methods, then determine what age group is best suited to the kind of book you want to write.