The materials are simple, the process easy- and fun too.
Have the kids color, pressing hard to solidly fill the space. Lighter, brighter colors work best. Or make sure to have contrast if you use darker colors. Once the whole paper is covered, crumple the paper a few times. Smooth out with your hand, being careful not to tear the paper.
Next, roll black liquid shoe polish over the whole thing and wipe with a paper towel. (It's the kind that comes in a bottle with a sponge applicator on the top.) Once it's dry, buff again with a paper towel to give it a bit of shine.