Parents have the choice of handing smartphones to their children. If cognition and future is that important, surely a parent wouldn't wait for government intervention

Its no longer an problem you can address on an individual basis. It affects whole communities, classroom as a whole, etc.

Teach a classroom of comprehensive children and then come back and say parents both want to and are able to control their children all of the time.

Yes, and that choice is far less successful if their children's friends all have smartphones already.