Hydrofluouric acid should do the business.
OK, I've given in an asked Wiki what plasticine is:
"Plasticine, a brand of modelling clay, is a putty-like modelling material made from calcium salts, (principally calcium carbonate), petroleum jelly and aliphatic acids."
So a strong acid will attack the calcium carbonate. Concentrated sulphuric acid should also oxidise the petroleum jelly quite effectively as well as the aliphatic acids which I think are just acetic acid and its longer carbon chain relatives. So I'd have thought your first choice would be fine.