Keeping the computer in the living room doesn't prevent them from stumbling on bad stuff.
The purpose of a good filter is 2 fold:
1. prevent kids from stumbling onto bad stuff.
2. prevent kids them from using the internet to be curious (the parent is paying for the connection and has a right to block or allow whatever they want).
A customizable filter is best:
k9 (win/mac). Customizable = if blocked it allows the user to invite the parent to override the block, (and add the site to a whitelist). This way, the kids can still access blocked items.
So who determines what gets blocked? With a customizable filter, it is the parent, and because they are paying for the internet connection, they have a right to control what comes in on the internet connection. When the kids move out and go to college, they are old enough they can make their own decisions.
However, I haven't found a customizable filter and free for linux yet. If you have an extra computer with 2 network cards you can try
untangle os a firewall os with content filtering. I will try the opendns suggestions listed below, because it sounds like you can choose what categories are blocked and which aren't.