Re: LAN Usage Monitoring Tool.
I did a google and track4win sounds promising:
http://www.track4win.com/Track_Internet_Usage.asp
Here is my argument with the internet and kids though. They are going to find a way to view what they want to view...even if it means at a friends house. If you teach them a "healthy internet usage" regimine they will be a little better off. You can restrict there internet usage to when you are home using your routers built-in functionality as well.
I got myself into a lot of stuff on the internet as a kid and I still do as an adult. One search leads to another and lets be honest...everyone is curious.
I would install the program, let them know the reason for installing it, and than prove that you are serious by "reviewing" the sites they've visited with them wether there appropriate or not. That way they know they are being watched and they will be less likely to abuse there priveledges.
You can also request any passwords they have for accounts, e-mail, facebook, etc and periodically log-in. If I were the kid I would be a little ticked off at this but it sonuds like they have already done the damage to prompt your monitoring.
If you gave me more specifics as to what exactly you want to track I could probably find a more suitable application. If you are simply looking for when and where they've been on the WWW no biggie...if you are looking at tracking keystrokes it gets more complicated.
If there getting into cam stuff I would almost change the way they login. I would make myself the admin, and lockdown any cam hardware so it's unavailable to them (unless I give them access) basically taking control of your network and becoming your own administrator.