When the word came up in a read-aloud I was doing at school, I asked the students if they were allowed to use "heck", and most said yes. (You can always count on Roald Dahl titles to provide teachable moments. "While this author thinks it's funny to use the word fat/stupid/heck, we know that it's not appopriate for us to do so, right? Right.")
What counts as swearing? How picky should we be? Maggie has brought home "What the heck?" and "Holy crap!" since she began kindergarten last month. I allow the former, but not the latter.
There are also the "I-didn't-really-say-it" partial expressions that children at school love to employ, like "Holy -!" and "Son of a -!". I know one student who is partial to saying (word for word, as he censors himself) "What the BEEP?"
This of course is different from innocent fouls, like when my learning-to-talk nephew received a new watch, and wasn't able to pronounce things quite perfectly when encouraging others to look at his clock. Though I don't think his parents can plead innocence regarding the time he commented, as he noticed a car turning but didn't understand it had an advanced green, "Look at this stupid bastard turning left on a red!" (He was four.)
I encourage the kids to use proper names for parts of the anatomy, but I sometimes turn a deaf ear when they are playing together and using silly expressions like "poopy bum". Kids have to have some fun, especially if I can pretend I didn't even hear them. However, I certainly don't want them to refer to anyone's "poopy bum" when greeting Monsignor Mike on Sundays. A lot has to do with the context.
To date (and this is subject to change) I have yet to hear the f-word from either girl (let me know if you have!), and I would wager that they have never heard it in our home. (I'm not saying I've never SAID it, just that they've never HEARD it.) That seems like the worst one to me, but really I suppose it's the Lord's-name-in-vain ones that we should be most concerned about.
That said, I am often heard exclaiming (in adult company) "Oh my God!" about matters that certainly aren't worthwhile of the Lord's consideration, and in those cases I don't feel that I'm "swearing". However, I wouldn't allow my daughters or students to use that expression.
What's allowable in your house? Do you have to watch what you say in front of the kids, or do you always keep your language clean? Are there different rules for different circumstances? I'd love to know what you think!