If you’re REALLY interested in the answer to this question, you should consider taking my home study course in writing picture books, Just Write for Kids. But here are a few nuggets:
Good picture books usually feature a compelling protagonist – either a child, or a child-like hero – who is engaged in solving some problem, and who learns something in the process, so that by the end they have somehow changed or grown.
The art advances the story as much as words do in picture books, so you have to be careful not to write what the art will show, yet also to craft a compelling and active story that will lend itself to visual interest and story progression. You want to avoid scene after scene of “talking heads,” for instance.
The best picture books are very economical when it comes to words. The goal used to be 1000 words… these days, it’s a lot less. So one needs to edit and streamline the narrative down to the fewest possible words, artfully chosen. Certain things are helpful in this regard, such as avoiding a lot of exposition and focusing more on good verbs than adjectives or adverbs.
And in the end, great kids books offer hope. Even if the subject matter is sad or poignant, there is usually a spirit of optimism to take away from the story. That’s one of the reasons they’re such a pleasure to read – and to write.