Know the age group of your target audience. Write appropriately for this age group, but be careful not to talk down to young readers. Children understand better than we sometimes give them credit for. Challenge young readers by presenting new ideas and concepts to them and teaching those ideas and concepts by weaving them into a lively, fast-paced story.
Create characters children can identify with. You don't need to make your characters human. Animals are popular in children's books, too. The key is to give your characters traits children relate to and put them in situations that are familiar to children.
Engage the imagination from the start. The key to a good children's book is to gain your young audience's interest from the first page. Young children have great imaginations but short attention span. You want to involve them in the story from the first page. The best way to gain the attention of your young readers is to present a problem they want to help solve.
Write straightforward, without a lot of adjectives and adverbs, especially if your target audience is very young. Short, active sentences are best when it comes to holding a child's interest.
Think visually. Many books for children contain illustrations. Think about the words you write and how they may lend themselves to illustrative ideas.