Glubble Family Edition is a free parental control suite for Firefox. I just installed this for my kids who are getting old enough to surf the net by themselves.
They start on a homepage with pre arranged sites that they can visit, and then you have the option to add other sites to the "Family Glubble".
The best idea here is that if one of your kids tries to access a site that is not approved, a pop up window comes up, and before they can view the site they have to email you and gain access.
This comes in handy with a 7 year old that knows how to spell!