I save my son's broken crayons. When I accumulate enough of them (and it doesn't take long), I bake them so they melt into a large blob.  First, I line a muffin tin with a muffin cup or two.  I used paper, but I think foil is prefered.  Then I put the crayon bits in there (paper wrapping removed, obviously).  Fill each muffin cup no more than half full. Put in a 200 degree oven and bake for 5 minutes or until the colors have melted together. Watch this carefully because you don't want them to start smoking.  Take them out of the oven and let them cool in the pan before taking them out.  Then your child will have a multi-colored crayon to color with.  For an added bonus, you can add glitter to the crayons before baking.