http://www.pscode.com/ prehaps?
If I ever found out my parents put a key logger on my computer, they would have one less functioning computer (and it wouldn't be mine) ;).
But really, why would you want to monitor your kids IMs? Why don't you just go the whole 9 yeards and watch what websites they goto? What data they transfer over the internet? Why don't you tap your own phone while your at it? Hell! Why not just 'bug' your kid?
Being worried about your kids IMs is like saying myspace is a horriable place for kids. Just doesn't make sense.
I mean...what are they going to talk about? Whos the hottest kid at school?
"Zomg did you look at the rear end on her in math 101!?"
If they find out your logging it, they will find a way around it. Every neighborhood has a hacker-wannabe-know-it-all-computer-programmer-kid who knows a thing or to about computers and will figure out how to get around your IM logger (or a keylogger if you write one). So, best advice is just don't waste your time.
Thats just my take on it...
Btw, check your PM on something intresting.