Doesn't sound too hard until you try to implement it. There are at least six different rating systems currently active, each with their own criteria for a good or bad site. The largest self-rating effort so far is being sponsered by RSCAi (Recreational Software Advisory Council on the Internet). Because this is volunatry, they have only been able to get 43,000 out of the 1.5 million sites to self-rate. Less than 1% of the web(2).
Not only are they not getting much response, but self-rating also runs into the problem of authors rating their site in a manner that does not agree with your values as a parent (1).
Rating systems also rely on the skills, criteria and honesty of those doing the ratings (3). Anyone can purposely rate a site incorrectly any will not recieve any punishment. That brings up the questions of making rating systems a statutory requirement but many people are attacking the idea because they see it as potential infringments on our liberties(3).