In response to people saying to not provide the filter, I would argue that you have to provide a filter, for no other reason than to cover your own butt with respect to the parents of your intended audience. Just make sure it can be disabled by the user. By implementing a profanity filter (albeit an imperfect and totally optional one), you can say that you've done everything expected of you to protect the sensibilities of your younger audience.
By making it possible to disable, you discourage users from trying to circumvent it using clever punctuation or substitution, since people who favor that sort of language will immediately disable the filter on their own computers, and will have long since forgotten that a filter even exists.
With that understanding, don't worry so much about the implementation. It doesn't need to be foolproof (which is good, because it can't be foolproof), but it should be relatively complete and as un-intrusive as possible. That is, you wan't to make sure you don't make the "clbuttic mistake".
The implementation can be extremely simple -- get a word list, and replace any words found in the list with asterisks or something similar. Best to search for whole words only, as well.
As for a word list, that's easy: http://www.google.com/search?q=profanity+word+list
Remember, it doesn't have to be all-inclusive, it just has to be representative of a valiant effort on your part to protect the children.