I disagree. I think if you aren't out and about signing up for free crap and clicking on want to meet up tonight ads, then you are farely safe from the malware and crapware that is Myspace and/or Facebook. Goto 192.168.0.1 or whatever your router login page is. Leave the username blank and for the password it's: password as the default for NetGear routers. Then there is a tab for firewall/page block settings. You can input certain addresses be it URL or IP's to block from and to your router.
Really though?
Get netnanny or get a machine-side keylogger to monitor your kids' activity. Seriously, if anything it's the people out there talking to your kids you should worry about, not stupid old myspace or facebook ads. (To be rid of the ads, only let your kids use Firefox and install the Firefox add-on Flashblock and that will remove 80-90% of the ads on facebook and myspace)
Damn it's good to be back in training.
-Concordedly