When my son was about three and into Spongebob, my wife and I made it a point to use Spongebob-appropriate curses. Something drops on the floor: "Barnacles!" Something breaks: "Tartar Sauce!"
He actually picked that up and I heard him go "Barnacles" a couple times before he caught on.
He's now 9, and I wonder whether I could get away with trying to slip him "Balrogs" as some kind of super-offensive swear word that is really reserved for truly terrible circumstances...