I naturally diverge and keeping ideas simple is difficult for me. I end up editing and cutting half my manuscripts because they have gotten too complicated and I have lost the essence of my story.
So, now when I start a picture book I decide right from the start the “theme” of my book. The theme should be able to be explained in one or two lines. What is the essence or heart of your story? This is your theme. The problems your character faces in the story will naturally develop from the theme. While writing I keep one or two lines explaining my theme right next to me. This way I do not waiver from my theme.
When I critique other people’s stories I watch out for the story getting too complicated. Is there a clear theme? How is the theme developed? Does the story stay true to the theme?
There can be a side story in the illustrations of a picture book. But this is a small, maybe humorous story line that is an extra spice not an interference with the real theme of the story. I saw the Bayeux tapestry recently and this was a perfect example of a side line story that was illustrated on the borders of the tapestry. There were images of peasants sowing grain and humorous animals doing strange things in the borders. This added to the original theme but did not try to compete with it.
There is an elegance and beauty in simplicity: just the right number of words, just the right number of brush strokes and just the right number of ideas in a picture book.