wow, I sure am glad my dad is completely tech illiterate...
Anyway, my suggestion would be to just limit connectivity after a certain hour, seeing as late nights (and sometimes before you get home) is prime time for well... you know. I saw where i could adjust these settings in my router, so you might want to see if your router has options like time limiting. But if you limit their time it might make em realy upset(at least it would **** me off really bad) I would just check in on them from time to time(make sure you step softly) because nothing in the world would be more embarising than having a parent walk in while your...you know. And if you choose to just check on them, make sure you never do it at the same time often. It wont take long before they have you timed. Theres also ways that you can get history reports sent to your computer(my freind says his dad could do that but i don't know how) Anyway i hope you have good luck with this problem. If you just keep an eye on your kid than you'll both be happy.