Look back to your childhood (ideally, between the ages of 3- and 10-years-old) and recall your interests during that time. The passions of your youth serve as a compass while writing your story, and help you find age-appropriate subject matter.
Go to your local library or bookstore and review current popular children's books for different ages. Speak with the manager or head librarian about your interest in writing children's books. They might know of workshops or other opportunities available for blossoming children's writers.
Decide which age group you're looking to attract with your book. Younger children will be more interested in picture books, while older children will gravitate toward fantasies and graphic novels. Outline your story and begin writing.
Revise your draft multiple times, and ask friends and family to review your story. Once you're comfortable with the content, let a child read it and ask for feedback. Ultimately, the only opinions that matter are the ones of your audience.