on Jane Friedman site:

To most readers, picture books appear to be simple stories, easy to write and short in length. New writers often enter the world of writing for children by attempting to write picture books rather than longer form stories, only to discover that picture books are not easy to write at all.

Good picture books are complex and layered, structured as carefully as a novel yet linguistically closer to a poem. If you are a picture book author but not an illustrator, you need to know how to make a text that will allow room for the illustrator to bring their vision to the work and yet create a complete story.

Protagonist, antagonist, rising and falling action, arc of change, emotion—all of these must be developed in a picture book, and generally with a word count of under 500 words. The words used must be simple enough to be understood by the youngest readers yet engaging enough to entertain adults.

Plot arc is one facet of picture books that can be almost invisible. Yet like all great stories, the arc of character change must be present, and must be driven by cause and effect and not a random string of events.

For an example, let’s study the plot of the picture book Big, written and illustrated by Vashti Harrison. (For you writers who do not illustrate, I have concrete takeaways.)