Both. The same as for other materials we don't want kids to access, like alcohol. We can't expect parents to always be watching their kids. That's not how societies have ever worked.
But what I'm actually questioning in my comment above is effectiveness of the technology solution proposed at the device level.
It's effective insofar as the parents secure the device. If it's a general purpose computer, and the parent forgets to lock the bios, kids will just live boot into Ubuntu or some other OS and do as they please.
Or they may install keyloggers (including hardware loggers) to get the parents' password and then go update their account.
Certainly this may help hinder them, but it won't take long for them to learn the basics of curcumvention, and the cost is regulated speech for OS manufacturers.
But what I'm actually questioning in my comment above is effectiveness of the technology solution proposed at the device level.