Writing a great book for children requires that you know the specific age group you are targeting. This will help will basic literary elements in your book, such as diction, imagery and tone. You will not be using an extensive vocabulary for a book aimed at young children, nor will you be focusing too greatly on imagery, since the illustrations in the finished product will help tell the story through pictures.
Many children's books feature animals as characters, such as in "Arthur," "Spot" and "Skippyjon Jones," which feature an aardvark, a dog and a Siamese cat respectively. Anthropomorphic characters are popular because children are drawn to animals and find stories involving them more attractive. Children should be central characters in children's books, where parents and adults, if present at all, are guides, advice-givers or even somewhat foolish characters. Your book will be more successful if children can emotionally relate to the characters.
Just as children should relate to the characters in your book, they should be able to relate to, or at least understand, the conflicts and events in the plot. Writing about important milestones in a child's life, such as the first day of school or getting news about a new brother or sister on the way, is one way to relate to children when writing the book. If you have children of your own, think about the important events in their lives, or think back to your own childhood to draw inspiration for the plot of your book.
One of the most common themes in all literature, not just children's literature, is the triumph of good over evil. If you pit a heroic figure against the "bad guy," you will give the child someone to cheer for. Not all children's books involve a moral, but teaching a lesson through your book is one way to write successfully for children. Writing your own fables or fairy tales that teach children something about values or morals can help you become a better children's writer.