My suggestion would be to have the kids bring in a used plastic / HDPE cutting board if they have any to spare from home.
Alternately, see if you can find a local building goods shop which has inexpensive 4’x8’ panels — envy the Midwest having: http://www.menards.com/main/p-2246284-c-14048.htm
Apollo just posted a design for cutting up an aluminum can on a Nomad: Recycling with CNC: Soda POP Robots — have the kids design something on a similar size / scale to cut out of a can?
Safer, but equally accessible would be large plastic lids from food containers such as coffee.
I’ve got a back burner project of taking my caliper design: https://www.shapeoko.com/projects/project.php?id=154 gets cut out of a broken CD case — those should be easy to source (an even cooler, but different idea is to make a CD case into a sextant). Another option are used / discarded CD and CD-Rs:
http://blog.cnccookbook.com/2011/01/19/cncd-art-from-old-cds-and-dvds/
(though that’s back to toys)