I was thinking of making little fingerprint ones for grand mothers gifts, slim thread could attach them to their mobile phone as a phone charm...possibly...if they didn't break!
I do want a real silver one made of DS for our xmas tree though on a star shaped charm, I think it makes a nice long term decoration keepsake.
clay, prob various different methods, here's one:
http://clayrecipe.net/jewelry-clay-recipe/you need the surface to be dry enough for the hands to be pressed in to get a nice 3D print but I reckon a bit of trial and error will come up with the right mix, maybe a dusting of flour over the top from a sieve to prevent the hand sticking in the clay and ruining the print shape as it's removed.
Another decoration that's nice for LOs to make is hand print trees. If you have space you can use wall paper rolls (backing paper works well) taped together, into a big rectangle on the floor, then an adult can mark out a rough out line of a xmas tree. All the LO's do lots of hand (and foot?) prints all over it in variety of green tone paints. When dried it can go up on the wall and be further decorated with other paper baubles, drawings, tinsel etc.