You don't enforce that, the owner (or the owner's parents, etc) of the device set that policy. MDMs can all already do this, there just needs to be a more user-

You don't enforce that, the owner (or the owner's parents, etc) of the device set that policy. MDMs can all already do this, there just needs to be a more user-friendly/consumer focused MDM to allow parents to control their kids devices. Just have it warn "Out of country sites may not follow your device policy, do you want to block them (Y/n)?"