C.C.
You should look into this stuff:
It is a polish-type stuff that you can brush onto children's nails (called "Mavala Stop") to prevent nail biting and thumb sucking. I assume it is safe, but I would look into it further before putting it on your child's finger nails.
http://www.amazon.com/s.html/103-###-###-####-###-###-###...
You might be able to buy it at a drugstore? If not I know they have it on amazon and ebay.