Any sufficiently motivated intelligent young teen can get around just about any monitoring software that lives on that PC.
All they need do is power down and reboot from a freely available Linux LiveCD. Your monitoring software is now not running. They can do whatever they want.
So....you have two solutions.
1. Visual screen monitoring....the PC in a common room in the house with the monitor facing where anyone can see it
2. A whole house firewall. All network traffic flows through that. You can monitor after the fact and block categories of sites or specific sites.
This requires a specific device....either a really good router, or a limited use PC, running a Linux firewall. I have one. It sits in the corner, between the router and all other devices, and blocks whatever category of websites I choose. Porn, shopping, whatever...if you do not know the password,it does not allow that traffic.