Comment MS Family Safety (Score 1) 646
I bought my niece (12) and nephew (11) laptops for Christmas, and I had the same problem. If you're running Windows as well, I found the answer in MS Windows Family Safety. As others have mentioned, a filter isn't really a replacement for parenting. I fully expect that when the boy's hormones start running wild he'll find a way to see boobs, and I'm not losing sleep over that. This (and MSE, and no admin access) keeps them reasonably safe from accidental malware installs.
It's also nice for time limits. Set a time to keep them from getting stupid on YouTube all night. They're told they'll be logged off 15 minutes prior, and they're logged off at the appropriate time. Their login is basically not functional, but you can still log in with your account. The filters are nice too -- you can manually white/blacklist sites (and occasionally some strange things are blocked for no reason -- Google searches will sometimes be blocked even though safesearch is forced on) or use their categories. The girl is currently banned from social networking as she went batshit crazy on Facebook. This wipes out FB, Myspace, etc. It also kills all webmail, though they have Google accounts (on my domain) and Google is whitelisted for that.
You can configure and monitor everything from anywhere with an internet connection. It's not perfect, and again not a replacement for parenting, but it's "free" (with a valid Windows license) and definitely "good enough."