Usually it takes a series of really good posts for someone to command my respect, but PerryLynch managed it in one.
Second thing: I'm not a parent, so maybe I know nothing about this, buuuuutttt ... I have to say that I'm in agreement that a keylogger is not the right tool here (at least based on what the OP has told us).
Prevention (either via personal communication, Net Nanny type things or Parental Controls) seems to me to be the right way to go. Speaking only as someone who remembers my youth VERY clearly, "gotcha" programs and the subsequent punishments did NOT go over well with me, and were not only ineffective but actually ENCOURAGED further deterioration of the relationship. That's my non-expert 2 cents.