I call this project, crayon batik. It's super simple!
We melt peeled crayons in tuna cans and then paint the melted crayon onto a fabric drawing.
I use old white sheets to create batik work on. I find illustrated kids books provide great inspiration for imagery. I have the kids draw onto the fabric using sharpie pens, this provides a coloring book style image to fill in with color. We iron off the wax, between newspaper, when we're done coloring.
The fabric is left with simply the dye from the crayons. This is always super successful!