What to do when your 5 yr old talks like a drunken sailor?

The Boy got off the school bus a couple of weeks ago and announced that the Girl had said “the F word”.

When I recovered from the shock, I asked him which F-word?  There is only one, of course, but hope was springing eternal in this human breast that there might be some others: flip, perhaps, or finance.  I used to get away with “frog” as an explanation for my own outbursts when they were too little to copy me (as they furiously absorbed it anyway, damn sponges).  Also, the “S-word” is, to them,  “stupid”, so theoretically the “F-word” could be – oh please God let it be – Flabbergasted.

“Fuck”, he said.

Oh.

Once I got over the shock of hearing this from the mouth of a babe (he'll always be a baby to me), I spoke sternly to the Girl, telling her that it is unacceptable for a 5 year old (or, furiously digging my way out of the hole I'd just made, a 6 year old or a 7 year old, or anyone who is not a very very angry, and later remorseful, grown up) to say that word.

“A_ told it to me” she said. A_ is older, and more worldly, and the recipient of much awe from the Girl.  

“Well A_ should not be saying it either,” I responded, then lectured her some more, until she looked suitably mollified, and gave her some crap punishment – no after dinner treat (so, just like every other day then), or similar – and dragged a pinky-promise out of her that that was the end of it.

(As an aside, the Boy then asked, in one of our more surreal conversations: “What does fuck mean?”  I gave it a moment’s deliberation, then told him – figuring that it would mortify him into never saying it.  This of course back-fired.

“Seriously?  That’s what it means?”

“Yup.”

“So that’s what grown ups say when they want a baby?  “Do you want to fuck?””

At which point I realised that I had just opened a can of very slippery worms indeed, who were wrapping themselves around my throat, making breathing difficult.)

Anyway, cut to today - different day, same story. They get off the bus and the Boy enlightens me as to his sister’s bus etiquette (or lack thereof).

So she was sent to bed early, her treat for tomorrow’s lunch has been rescinded, she’s not allowed to bring any toys into school for a week, AND – worst of all – I’m telling her father.  (Not to mention you lot.)  She is, as we speak, crying crocodile tears in bed, and feeling very hard-done by.

So, dear reader, I would like your advice / guidance / tales of dealing with similar.  I HATE hearing young kids cursing.  She is FIVE.  It’s appalling. Also, there are little  - and big – kids on the school bus and I really don’t want my child to be the one teaching them to curse.  I can’t keep punishing her every time it happens (of course it’s going to happen again), because it seems to have little effect – other than make me feel as if I’m taking proper steps.  What have you done when confronted with a potty-mouthed youngster?  Does anything work? Am I on a hiding to nothing? 

I’m... Flabergasted.  Help.