Most home gardeners extol the virtues of compost and composting. Here is an easy-to-follow “picture book” that tells what it is and how to do it.
http://aggie-horticulture.tamu.edu/kindergarden/kidscompost/CompostingForKids.pdf
To give kids a hands-on experience with what compost is and how composting is done try one or more of the home/classroom activities described here.
http://www.cvswmd.org/uploads/6/1/2/6/6126179/do_the_rot_thing_cvswmd1.pdf