Here's a selection of books and poems to run with:
Arthur's Nose shadow puppet show
"The Dog and His Bone" shadow puppet show (patterns from Judy Sierra's Fantastic Theater: Puppets and Plays for Young Performers--amazing book!)
Mother Mother I Feel Sick Send for the Doctor Quick Quick Quick as a shadow puppet show or just read the book
"My Shadow" from Shout! Little Poems that Roar
"Shadow Wash" prop story
You can also hang a plain sheet up and let the kids practice making shadows in front of the audience. Photo opp!
A fun activity for slightly older kids would be to team them in pairs and have them take turns pretending to be each other's shadows. We used to do this in drama club.