Because you can't police who is using the device at the other end. You're authenticating a client device, not a person, and that doesn't keep children any safer

Because you can't police who is using the device at the other end. You're authenticating a client device, not a person, and that doesn't keep children any safer from abuse - if anything it means they have an incentive to keep their use of a thing secret for fear of punishment, which makes them more vulnerable.