I recently came across a useful tip by Hannah Holt on the 'must-have' elements of a picture book:
According to Hannah a PB must have:
Desire: what the character wants Motivation: why they want it Tension: what separates them from their desire Stakes: consequences of the failure
I looked at two picture books that I am currently chewing on and ran a quick checklist. It was quite useful! For instance, in my WIP - Pigs Don't Yoga, a cute story about Pigs that are forced to learn yoga by their farmer, I noticed the tension was very low. I worked on the text to build the the tension and it really helped make the manuscript feel more polished.