I'm not saying parents shouldn't try.

But like your examples, it's not guaranteed to work.

Install a DNS filter. Firefox circumvented that? Smart kid :)

Censorship circumvention might be a good skill to have in the near future.

Well in the past there were laws banning the selling of cigarettes and alcohol to minors. Selling alcohol to a minor can cause you to lose your liquor license. With much of the online content being free and not involving a physical product it has become significantly harder for a parent to protect their children.

If one kid is able to bypass the system it means it's zero percent effective. Same thing with alcohol, and cigarettes. Especially if it means I have to show my ID to buy those things.