If the app on the kid's phone knows that the phone is registered in the UK, how would a VPN circumvent it?
"But everyone uses their phone all the time!" Yes, and everyone will be worse off for making the obviously worse choice.
Kids do.
Sure if they want to circumvent and go home and use Dad's laptop to cyberbully or send pictures of their wang they probably could ...
I suspect that the number of kids using not using an app on a device that is aware of its locality is a rounding error.
Tesco* phones on some network's family plan must be 95th percentile.
(* other high street retailers are available)
Yes.
On at least Android phones, and I'm pretty sure also on iOS, location access is a user-controlled permission that's not necessarily granted to any given app. There always leaks, but are you going to require commercial software to play cat-and-mouse games to get around system security settings?
And some, possibly most, "social media" can also be used without apps.