A general comment on content filters....
Anyone with children browsing the Internet on a computer should make sure they are speaking to them about what is and is not acceptable to do or look at. Much like the talks you should have with them about drugs and other potentially harmful things.
So, a content filter is really just the way parents can say with actions that they really mean it. It should not though be a substitute for the conversations and understanding of what should and should not happen.
The reason is, all of the content filters fail. They either block too little or they block too much. If the child is smart, and all parents wish their child to be very bright, then a filter will not stop them from seeing something they want to. That being said, as was mentioned in the last post, I have heard better things about NetNanny than some other ones.
For very young kids, there is another alternative as well. Instead of opening up the Internet to them filtered or unfiltered, you instead select exactly what they can see. Browsers can be set in high security modes where they will only open a predefined list of sites. All others are blocked. So you can hand pick which sites to allow and set up bookmarks for the kid.