If you want to do the stomp thing, and you already have a PA, than you should also have some mics. You have a desktop or laptop? Laptop, go with a USB audio interface. Desktop, I'd go with a PCI based one. Try
www.midiman.com
Mic the kids, mix on you existing PA (with headphones if possible- mix for what sounds good in the phones, not in the house), take a left/right out of the board to the audio interface on the computer, fire up something like PG Music's Power Tracks, hit record, and go!!
If you're talkin' kids, get a copy of Acid Pro and a beat box of some kind and let 'em do some hip hop/dj/remix stuff. That'd be probably easier than trying to mic the stomp thing up.........