It truly was a test of patience on my part...teaching shading to 9 year olds who call it "coloring". Oy. In the end I think we all learned a little something, and their artwork definitely shows the beginning stages of VALUE! Score!
If you live in an area where shells are NOT readily available, I've seen some shell packs in Sax Arts catalog. In fact, I think some of the shells we used were from that catalog! Each was assigned a shell and we drew in contour on day 1. On day 2 we made a 5-shade value scale and then used that to parallel into our lesson on shading and value.
I entered one of these at the last minute into a community art show over the weekend and it won a first place ribbon! I love it when things work out like that!