I have to say, kpac's response is the most sensible here.
really, any sort of "parental controls" are really just a delegation of parental responsibility, the very same thing that's been happening for years with television. And the parents should keep the PC in a location where they can see it- or in a high-traffic area. Kids do NOT need a PC in their own rooms until they reach a certain age/maturity, at which point some basic blocks could be placed on the router, but by that time the parents should feel comfortable with the child having a PC in their own room and using it unsupervised, anyway.
By placing "parental controls" on a kids PC and letting them have at it without supervision is really just a challenge put to them to get around the filter, and, let's be honest- kid's can be very clever when they need to be, and the fact that most kids are better with the computer then their parents (in fact, I've even heard of such parents being so illiterate they actually got the kids to install the blocking software- yeah, they certainly had an incentive to install it properly
).