For my kid's account on OSX El Capitan, I've turned on Parental Controls, with the Web settings to "Try to limit access to adult websites". This causes non-adult websites to fail to load. For example, edx.org works for a while then inexplicably, Safari/Chrome can't access it. The same happens with nytimes.com. ping and traceroute to these sites eventually fail also.
Why?
Best Answer
It's not a perfect system because there's no way for any filter to know exactly what the content of a site is or whether it's an "adult" site or not. This is from Apple's site explaining the filtering:
So there's no way you're going to be able to do perfect filtering.