Draw the outline of a hand, very lightly in pencil, and then began drawing lines freehand across the paper. When part of the hand is in the way of the straight line, have each artist "crawl over" their finger or hand with curves and arcs. The optical illusion becomes very obvious if the lines were close together and the curves fairly strong.
A great thing about this project is it is JUST acrylic paint black Sharpies (love me some Sharpies) and scrap booking paper (Oh, and prepared canvases, Modge Podge and a Cricut machine) I think the last 3 items add a little cost - don't you wish schools had Cricuts? Ellison Machines just aren't the same but could do in a pinch.
Water color crayon resist artwork, an Original watercolour painting by Kirsten Bailey, but could be adapted for a children's art activity. Great for a fall wall hanging or spider picture for the fridge!