If you have Vex kits, you could make a simple game with 3-4 Vex robots, some balls and a ramp.
Each kid controls a Vex bot, and the arena is a circle/track with a "steep" ramp on one side. (30-45 degrees)
Small balls are placed around the "track", and the object is to herd these balls around the track. Each lap with a ball earns that bot/kid a piece of candy.
Place a large/heavy ball (think soccerball...or trackball.
) in the center, heavy enough that one single bot can't push it. However, if 2 or more bots team together, they can push it up and over the ramp, earning ALL kids 5 pieces of candy.
Of course, Vex "battlebots" also works. Pushing matches are fun....
If the demo is long enough, let kids form teams and make devices to help push balls/push other bots.