Character arc is something most editors look for, and lots of books have it. Because the scope of picture books is short, it's usually a change or realization; the situation is different at the end than the the beginning, and the characters are more experienced and smarter. Often it can be described as a lesson, but in a good story, it doesn't feel that way. For example:
The Recess Queen: Mean Jean bullies other kids on the playground until a new girl joins school and offers her simple friendship, and Jean realizes that's better.
Pinkalicious: Girl greedily gobbles all the cupcakes until she turns pink. When she turns red, she realizes she's out of control and eats healthy green food to turn back into her old color, which she is now happy to be.