Pff... As a parent with some experience with this thanks to a 2 year old who loves to explore what the buttons do while I try to get some house chores done when I'm not looking so before I know it she is changing the password on the laptop to some random set of characters, you simply can't monitor their adept fingers, quick feet, and inquisitive minds every second of every day - the best I can hope for is that she remembers all the little lessons I teach her so she doesn't get badly hurt while she is learning.
We've been to NickJr.com before so she could play Diego Animal Rescue or Ni Hao Kailan stuff monitored simply because of the aforementioned tendency to hit the windows menu key or right click and bring up a host of source code and settings related stuff, so I feel for those parents that might have had an older kid who doesn't want to be treated like a kid being babied all the time, so they leave em alone for a sec to find later they found their way to some internet adult content by way of a ad on a children's site.
What makes something adult content in my eyes? Anything an adult would have some trouble explaining to some stranger's kid (because who do you think they are going to tell what daddy or mommy said to?) why adults find this or that entertaining but forget to mention it took perhaps anywhere from 5-30 (or maybe never) to mature to a point where we don't feel a need to bring the subjects up just to feel rebellious and cool.
I'm going to take the approach as a parent of not sanitizing my kid's world but rather try to prepare her for it in smaller, easier to handle doses so hopefully there is nothing to rebel against later, only talk about openly and honestly so I know she is safe, healthy, content, and found some bit of hope and joy in life. She seems fairly happy so far, so I hope I am doing alright.