Set up a dummy WEP-secured AP and teach them how to get on. Check out http://www.securitytube.net/groups?operation=view&groupId=9 for a good instructional video. You'll have to provide a few Alfa cards though. Have them work in teams of three or four and you won't need many.
Seriously. They will learn not to run WEP on their own APs nor to trust WEP APs in the wild. And since most people don't run WEP anymore, you aren't really setting them up for a life of criminal hacking. But it is just devious enough to entertain kids (and some adults).