I don't have experience with Symatntec Online Family, so I can't speak to that. OpenDNS is far too easy to bypass in my opinion. It only blocks blacklisted domains. For example, depending on your settings it may not be blocking Reddit.com or other similar content sharing sites and it is therefore quite easy to find porn in any of the numerous adult sub-reddits. Sports Illustrated is another good example. If SI.com isn't being blocked, you would still be able to access the swimsuit section of their site. The fact that OpenDNS filters all devices connected to your network is the only advantage it has over K9, in my opinion.

K9 will need to be installed on each device, but it is more difficult to bypass. It also provides you with detailed logs of websites visited and the number of times pages within certain categories have been requested. Of course, it will be difficult to find a solution that is completely effective. You'll be able to find porn if you look hard enough no matter what you're using, but K9 is a much better solution in my opinion.