Somehow we've not had the problem (yet). They get bored quickly and self-limit themselves, though we reserve the right to look at what they're up to whenever we see fit. Having lots of extracurricular activities might be helping, but my introverted, nerdy self spent way more time glued to a CRT as a teen. Hmm, maybe that's the secret.. having a nerd for a dad makes tech look uncool! :-D