As far as a simple project, lettuce is an easy crop. You can surface sow your seeds on moist Perlite & transplant them later to a system. Given the age of the group you are working with, they would probably like building their own system. A simple raft system can be easily made by purchasing products at your local dollar & hardware store. Below are some links to some systems made by forum members:

http://forums.gardenweb.com/forums/load/hydro/msg121151236631.html

http://forums.gardenweb.com/forums/load/hydro/msg0510515721118.html

Here is a PDF file in which lettuce is grown by preschool children. It looks like they used an opaque plastic box with holes drilled in it to hold the nutrient & lettuce.

http://www.chasque.apc.org/frontpage/aquafood/Pagina-Ingles/British-Eng.pdf

Mint is another idea. If you put root cuttings in Perlite & grow it hydroponically it propagates quickly.