Something that has worked with students who have limited hand control was powdered graphite. Ok this stuff is messy and ya might wanna use a mask but it works like a charm. Just sprinkle some graphite on large white paper and then have the student blow it into the shape they want or even use a stump either in their mouth or hands if they have the control. It isn't as fine motor as a pencil or other drawing tools and can give some pretty awsome results.