I *have* been crazy enough to do this kind of thing before
First off keep it very simple, and make something very general like birthday or friendship. I would design 2 cards, one more feminine and one more masculine and let them choose which they want to make (a guy might want to make a pretty card for his girlfriend or masculine for a guy friend and vice versa) Pre-cut everything you can and set up stations or maybe give them each a packet of what they will need. If you could use some stickers instead of stamps kids (even teens) seem to like those more and they are alot less messy for you to clean up after. Also don't bring your favorite stamp set or brand new inks pad because they will not come back in good shape;)
Hope this helps, have fun!