One word of warning about any browser based filtering solution; it really doesn’t take much effort to bypass these measures. Your kid could, for example, choose to use a portable Firefox install or the really determined could achieve things manually ( admittedly, not many are likely to be determined enough);
telnet http://mynaughtysite.com 80
GET / HTTP/1.1
They can then copy and paste the output into notepad, save as a HTML file and open in the browser. Not exactly graceful, especially if you want to browse an entire site, but still entirely possible.
Despite this, however, many browsers do incorporate basic filtering methods. These can be effective, but are reliant on sites conforming to certain standards. Unfortunately, the sites you are likely to want to block are the very ones likely to ignore these!
The adult industry appears to have no interest in serving their content to minors, and already incorporate robust mechanisms to prevent access by minors. The sites that pose a perceived risk are those that do not care who they serve their content to, who by their very definition aren’t going to worry about including meta-tags to identify their content as adult.