I found a couple of great gag gifts - they would work really well for kids, too. The Potato Gun:

The Original Harmless Squeeze powered Potato Gun. Just press the tip of the gun into a raw potato, break off a small pellet, aim, squeeze the handle and it will shoot the harmless potato pellet far across the room. You can get hundreds of shots from a single potato. Potato Gun size is 6 inches and simple to use. One Potato Gun per Package.

And the Fart Pen:

It's a real pen shaped like a finger and when you pull on it, out will come farting sounds. Great for that boring night of homework or maybe make your friends laugh at this very comical pen.