What material are those rollers made out of? It doesn’t look like PVC or ABS to me…
Looks like a fun demo for your team, and the kids in the picture 
We did a demo a few weeks ago and, near the end, spent more than 30 minutes letting kids toss balls into the hopper and dumping them all over them. They had a blast.
I most enjoyed – to a point – the parent that didn’t realize someone was controlling the robot remotely. She kept walking up the robot and kicking the bumpers, somehow thinking that’s what made it dump. I politely asked her to stop. 
Team 980 had similar fun on Friday (May 15) at the Space Day event in Riverside. The kids just loved when we’d spew a bunch balls for them to toss back at the robot.
Here they are trying to load up our robot:
http://i33.photobucket.com/albums/d83/dbrinza/SpaceDay_980_Shoot.jpg
Sort of looks like nylon.
They could be UHMW as well.
Should have searched before I posted. Here is Jon Jack’s detailed description of their ball system. Looks like sdcantrell56 was right, its UHMW.
My next questions would be what size tubing is it and where did you get it?
Looks like UHMW
It’s UHMW. The tubing is from McMaster (8705K74). It has a 2-1/4" OD. We machined aluminum hubs for the ends and reinforced the tubing with a .09" wall aluminum pipe. The pipe kept the tubing from deforming under tension.