> For instance, if a child buys their own device

Then the law can make it illegal to sell smartphones or computers to 12 years olds or we could just ask the parents to do a bit of work and ensure their children is not buying devices behind their backs.

The idea is to make it easy for responsible parents to give a device to their children and make it easy for legal websites to block minors from adult content. We can't get perfect results but good enough could shut upo the complainers and maybe we get them do things like educating parents on how to proceed when they gift a device to a child.