Before I share this with you - though a great idea, Sparks just don't have the wrist strength for this - Brownies and up would definitely be able to though, and it's totally safe.
So you need a bar of soap and a paper scorer. My local dollar store sells them in the scrapbooking aisle.
Use the scorer to carve into the soap. The scorer is dull, and won't hurt them if they miss or accidentally hack at themselves with it.. Soap, however can crack along lines in the bar itself, and if a girl has a hard time because their sculpture is cracking, she should try a negative image (carve out a shape in the centre, leaving soap around the outside edges).
Here's my daughter's sculpture - she's 1st year Brownie.