Both our girls do it too.
we don't make a huge deal out of it and just say that only mummy and/or daddy can say that... it's usually just the s word cause i tend to say it if i hurt myself or something like that.

They if they hear each other say it they pick each other up on it and say only adults can say that lol

i figure if i make a huge deal out of it it gives them the impression that it's this big horirble thing. Yes it's not nice to say, but i figure they will hear it from more people than jsut me...