You paint the child's hand with white paint and have him grab a shatterproof ornament (most people choose blue but it can be red or green or purple). You can find pictures on google. After the handprint dries, you (not the child) paint a hat and face and buttons on each finger.
If you are sending it home to their parents or to grandparents, enclose this poem:
These aren't just five snowmen
As anyone can see.
I made them with my hand
Which is a part of me.
Each year when you trim the tree
You will look back and recall
Christmas of 2013
When my hand was just this small.