There is no simple answer. A promise no government can keep
Brian, do you have kids? Doubt it.
We aren't talking about 5 year olds here, its pre-teen to teen kids. You can't sit and watch the kid all the time. Physical keys haven't been on PC cases for years (unless you have a server case). But you can do other things like remove the keyboard and power cord. But here's some reality checks for you:
1) I don't like people looking over my shoulder when I'm trying to work why would a kid.
2) Kids need some privacy to chat with their mate about that hot chick/stud at school. Can't do that with parents watching
3) More than one kid of that age in the house? more than one computer? can't watch them all.
4) I want my evening time to myself, or to continue my work, or to also be on a computer, not supervising net usage. My wife has a life too.
5) Maybe I don't want my internet line filtered at the ISP because I don't want my speed hampered or I don't care if I see the odd bit of filth on my own PC?
Install password protected commercial filtering on the Kids PC(s) and use a strong password on my own, and randomly check what they are doing online. The main problem is the parents who aren't tech-savvy. They just have to be strict with the kids time on the net and be clear with them that there will be lots of trouble if they are caught looking at things they know they shouldn't be. Other than that if you can't trust your kids to do the right thing, then they can't use the computer.