Kazoo is one of the easiest instrument that kids can play. It is also affordable enough to outfit the whole class. They come in different colors too. I think that providing the whole class with single kazoo each will be fun. It might be loud but I am sure the kids will have a blast playing this instrument.
If you are a music teacher and wants to let your students play a kazoo, you should look for kazoos at musicians friend. Even if you are just wanting to give some fun instrument to play with to a group of kids in the neighborhood, kazoo is a perfect choice.