Internet traffic can be controlled on the router, and then no matter what the kid does with their own machine they can't get around the filter if it's done properly. (The only way around is to connect to the neighbour's wireless, but that is easily detectable though not preventable).
The thing is very few parents would know how to do something like this - in fact worse, they don't even know THAT it's the best approach, and there are no commercial solutions aimed at the market - probably because parents will see £100 web filtering router against £20 software and go for the software every single time.
- How do you plan to identify your child's activity? I can spoof my MAC address--hell, I could make it look like the computer of another family member.
- What if the child sets up an SSH tunnel on port 80 of a remote machine? Then he could surf anything he wants and it will look pretty innocent--especially if he only uses it for say, porn. Are you really going to packet shape SSH traffic? Especially if I spoof my MAC address--then you're going to packet shape SSH traffic for everyone in the family?
- Even if you packet shape SSH, I can still tunnel HTTP through another server. If the server's proxy software rewrites the HTTP headers and HTML, it'll be pretty hard to identify whatever you are viewing as a porn site. Maybe your router software identifies pornographic images? What if my proxy converted all images to flash videos? I could even (non-SSH) encrypt certain sites to download say as, passworded ZIP files. While this would be a painfully slow way to browse, it's tolerable enough to put up with for a few videos...
- I hope the parents never plan to leave this kid out of their sight, because I could likely undetectably recover the router password from a parent's computer if left alone (even if password protected...)
- How do you plan to detect wireless connections to a neighbor's wireless if I change my MAC address? This would be even harder if I had an "in" on the other side--say another kid who sets up the technology in the house.
Sure, if you are diligent enough, you can block or detect any of this. The point is that many parents are looking for a quick, non time consuming fix that they can setup and leave alone. If you are a real parent who supervises their child's Internet activity, then there's not really much the child can do. However, if you think that setting up a router or installing some fancy software means you can rest on your laurels and not do your job as a parent you will always fail, no matter how clever the solution is.