Foam board is a polystyrene material used in framing pictures and making architectural models. Scraps can be cut with a utility knife, and used to make small easels for displaying work. Since the foam material is covered on both sides with paper, it can be easily painted and glued.
You can get scraps of foam board or foam core from picture frame shops. They throw away the smaller pieces, but they may do well for your easels. If you want to see an example, copy, cut and paste this address:
http://www.kid-at-art.com/icons/easlside.jpeg