If your kids have been doing any amount of drawing, you're bound to have bits & pieces of broken crayons.  Here's a project your kids will have fun doing (with your help) that will recycle those crayon pieces.

Get a candy mold from the craft store. This is going to be the mold for your new set of crayons. Stars or hearts or anything with a point will be a good choice, as they will be more user-friendly to draw with.

Collect bits of crayons, or break crayons into small pieces.  Choose colors that you think will work when blended together (it's really up to you).  Put the bits into an old baby food jar or other glass jar (something that you don't need, because you don't want to have to wash out the residue later).  Then, put the jar in the microwave.  Heat it in 30 second segments and watch it melt.  The melting time will depend on how much crayon you are using, so just keep watching and reheating until the crayons are melted. 

Be careful removing the jar from the microwave (obviously this part is only for adults).  Pour into the candy molds.

Then, put the mold in the refrigerator (or you can leave it out--it will just take longer) until it hardens.  Once hardened, you can just pop your shaplier new crayons out of the molds, and your kids can start drawing.

Note:  These make a nice addition to a birthday party favor bag.