I would recommend 6 inch VEXpro wheels:
http://www.vexrobotics.com/vexpro/mo...um-wheels.html
The 6 inch variety isn't very expensive but is still big enough to see them working well enough. Also a smaller wheel means less gearing.
I think either a modified KOP chassis or a Versa-frame with the clamping gearboxes would work great. You'd probably need to make a guard around the gears to avoid potential for harm.
You could also try a Andymark Nanotube chassis:
http://www.andymark.com/NanoTube-p/am-0809.htm