card_richelieu, I checked around and found this:
http://www.im-protector.com/
It seems like a decent option for you if you'd like to monitor your kids IM'ing. Only drawback is that it's not free. It costs $80 for the year but it does come with a 30 day trial. If you're not financially bound and/or don't mind spending the $80, this may be a good option for you to consider. Maybe try the trial out and see if you like it? Other than that, I'd listen to what the others are saying, especially educating your kids on certain matters in regards to IM. There's no security tool on the planet that can successfully combat ignorance to the degree that education can. Anyways, good luck.