Micheal, Get some base plate wax from your dentist. One sheet
will do just fine. Cut a piece from the sheet about 4 x larger
than you need for the print. Warm the wax until plyable and then
put the kids thumb in it 'til it gets cool. This will not be hot
enough to hurt the child. You can trim it out with a scalpel,
sprue, etc. Be sure to use a dental investment. It is so
accurate that it can reproduce the striations on a prepared
tooth that are left by the grit of the fine diamond burr.