I'm teaching kids to program using Ruby and the gaming framework Gosu/Chingu. For the sounds for their games I've been showing them http://www.bfxr.net/. It's decent, but the samples are limited and some of them are pretty cheap (check the explosion, it's like an explosion on a commodore 64 game).
Is there an easy resource kids can get the sounds they want? I'm happy to pay some kind of educational license for it.