Lesson stories always make good material for children's books. The key to writing a good book for children that teaches a lesson is don't preach. A moral at the end of the story is good. There's nothing better than a story written that entertains and teaches, but too many beginning writers want to preach the message of the story rather than let it unfold naturally. If the moral is clear, children will get it without having it force-fed to them.
Stories that feature main characters of the same age as your target audience being introduced to something are good stories for children. You could be introducing the main character to painting or how to play a musical instrument. Story titles such as "Bobby and the Magic Flute" should spark immediate ideas. There are many adventures you could put Bobby in that involve his magic flute. A book like this would be a good way to introduce young readers to music. A book with a musical theme might be published as an interactive book featuring instrument sounds readers can hear by pressing a button on individual pages.
Stories that introduce children to the simple joys of life are great fare for children's stories. Put your main character, whether a boy, girl or talking animal, on an adventure that takes him to exotic places, real or imagined. Children love to be taken on adventures. These types of stories are excellent for teaching children to appreciate where they come from by taking them on adventures far away and then bringing them home again.