We did a craft in Brownies once. I'm remembering this from 20 years ago as one of the kids doing it, so I hope I'm not forgetting anything crucial.
Take a sheet of wax paper. Gather some pretty leaves or something, maybe even cut nice shapesout of construction paper. Lay them on the wax paper, not filling it up, just a bit here and there. Grate crayon bits and sprinkle them on the paper however you want (ie, you grate them up and let the kiddos sprinkle). Lay another sheet of wax paper on top and you pass a hot iron overtop, not touching or you'll have a waxy iron, until it all melts together. It's stained glass! I think we put some construction paper around for a frame, and put a string on it to hang in the window.
I know it doesn't use a lot of crayon up with only a few kids, but maybe you can do it as a group activity for an organization if your kids are in something (sunday school, scouts, 4H, etc)?
If you have a slow-cooker with the warming plate thingy that the cooker pot rests on, you can do "stained glass" like my mom would do with us when I was little.
Cover the warming base with a layer of heavy-duty foil. Cut some white copy paper in half so that a sheet will fit onto the base. Let the kids color on the foil when it's warm enough to melt the crayons. Once they have it all covered with beautiful colors, lay a piece of paper on it. Use a wadded-up paper towel to press the paper into the melted crayon if needed, then lift off and let it dry. Wipe off the foil with paper towels between uses.
When you have all of your "stained glass" pieces made, you can either hang them in the window as-is, or cut stained glass "lead" patterns out of black paper to make frames to put over the crayon'd pieces. Either way, your kids will think you're the most brilliant person around