This would suddenly mean no more custom browsers, no more custom OSes, and I doubt they'd cater to the Linux and BSD crowds with this one. It's something the OSS community has been trying to fight for the last 4 decades. With a full-on government requirement this would lock you to the vetted platforms while letting anything other get in would be illegal for the site owners.
Not really, they'd just have to send the "I'm a child" header if the "I'm a child" flag is set. Linux could have /etc/childlocked set to 1 — a global setting instead of per account isn't ideal, but it would satisfy the law.
It doesn't mean any of that? The point is that the parent, not the child, is the owner of the device. So the parent can restrict the device they own before handing it off to the child (or the same with accounts on the same device).