So, you tried to limit the kids away from some sites?
YOU GOTTA BE KIDDING ME!!!!
I just turned 15 myself, a few weeks ago.
Theres no way you should, that brutal, away from sites.
When something is forbidden in that way, it gets more interresting to have a look at.
The only way to learn teenagers rules, is to make them understand that some things, you just dont do, and are illegal.
Internet filters a made for kids, that would begin crying if they ends up on a page containing stuff they would not like to see.
If it can be run, you can crack it. Remember? Teenagers arent stupid.
If it was my unkel/parrents whatever, i would just laugh, very much.
Your only chance to stop them from doing it, is to log everything, and show them the logs, if theres something that you dont like.
Embrassing the kids is the best way.