That works well for controlled devices like phones, tablets, and TVs, but it’s much harder on desktops unless you expect parents to become IT administrators.

What's wrong with asking the user on account creation and OS install?

It'd need to be vertically integrated from OS to user space apps, and restricting what can be installed.

I'm not sure to understand the proposed solution here, but it seems someone could just use a different web browser client who don't inherit these restrictions.