I noticed that my in-laws 13-year old kid spends all his time glued to their iMac. I know that his user-account has parental controls enabled, and his computer-time is limited. I talked with him about it, and he mentioned that "Those time-limits are useless, since I can easily bypass them". He wouldn't tell me how he does it, though.
It's your in-law's kid who's taking advantage of the time restraints and parental controls that I assume your in-laws put on their mac; the way I see it, it's not only their computer and their time restraints--it's their boy who is taking advantage of both. Therefore, before you take matters into your own hands, I would strongly suggest that you talk with them about the matter first; offer to assist them in finding a more affective way of "plugging the hole".
If they oblige, go for it!
Worst case scenario? If they not only don't oblige, but also refuse to do anything about it, then I think both of you guys need to re-evaluate the agreement you all agreed to so that their boy could stay at your house (am assuming that it is your house).
This assumption correct?