PENNY BRIDGES!
Actually I don't know what they are really called. How many pennies can you get on a 3x5 index card bridge before it collapses. I've been doing it for the yearly 3rd grade Math & Science Night at my kids' elementary school. (This year was the first year I missed it
-- it was the same night as I had to be in Atlanta for some little robotic competition...) Very popular; some kids we have to toss out at the end of the night. Even the parents get into it. Some kids got upwards of 200 pennies!
After the first year or two I made some U-shaped supports out of 2x4s nailed together to keep a consistant distance, otherwise the supports tend to get closer together. I also bought a cheap scale to measure how many pennies, otherwise you get little kids counting every -- single -- penny.
The secret, actually two secrets, is how you fold the card, and to stack the pennies very carefully.
Since you have only four groups of many kids (vs my lots of 1-2 kids plus parent groups), maybe you can enlarge the size of the bridge to a manilla folder size (cut into letter size?). Of course, that means more pennies...
Roger.