Via Tara Reed, host of the Art Licensing blog, I discovered this post on 25 Kidlit Portfolio Prompts: Expressions written by illustrator, Wendy Martin. If you create your own characters, you'll want to read this articcle.
Wendy brings up an important point: "When illustrating a book, the main character will experience a range of emotions that follow the story arc. Art directors will be looking at the sample illustrations in your kidlit portfolio to see if you can portray recognizable expressions, in a consistent manner for the length of a book. At a minimum that’s 15 images. Fifteen images with the same character being happy, sad, disgruntled, bored, angry."
Wendy offers 25 Expressions to help you build your kidlit portfolio and a list of reference books. Read the complete post for more great ideas. Then start cranking out those expressions.