Not it's location.
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.