This is not a problem that needs solving. It's hard to "accidentally" run across something really harmful on the internet. And run of the mill porn isn't. By the time your kid knows what's going on in those pictures the cat's out of the bag. My wife and I admitted to each other once that we first downloaded porn around age 10-11 (over our 2400 baud modems!) We turned out, by all accounts, normal and well adjusted. So did my friends, who were doing the same thing at the same ages. We're job-holding, child-raising respectable adults ourselves now.

My wife and I have a young daughter, and we can't see any reason to control what she does online. Bigger picture: kids don't really need to be protected from the world. They need to be taught right and wrong and how to protect themselves, but it's an enormously bad idea to try to sanitize the world to be more suitable for kids, instead of raising kids that are suitable for the world that actually exists.