Working with children you find yourself saving a lot of recyclables such as water bottles, cardboard, and all sorts of packaging. Here is a new one for you to start collecting, cheese wax. Its the red stuff on the outside of the Babybel mini cheese rounds, as well as other various cheese, and comes in several colors. It is great for creating stamps and as play dough.
The cheese wax can be melted or softened and molded. If you plan on melting it completely to pour into a container I would suggest using a double boiler. It not you can soften it in the microwave and shape it with your hands. We used to popsicle sticks to flatten the sides making our rectangle. After it has hardened use a toothpick, or a different child friendly device, to carve a design into the stamp. Then add a little paint or an ink pad and enjoy.