Today's kids are discouraged from walking around unsupervised, discouraged from partying together, discouraged from fighting and pranks.
So we take away their phones and kids what, sit around at home? Homes where the parents are phone users ie homes without libraries, without adequate crafts.
Phone-free lifestyles require a society-wide infrastructure, and we don't have that.
Invite your buddies over - your parents are nervously hovering. Hang out at the park - adults find you strange and might call the cops. Schools close at 3, malls gone, libraries full of homeless people.
What's going to happen is the rich kids will be fine, shuttled around from activity to activity. And the poor kids, the kids who used their phones as escapes from dull, grey homes - they'll suffer.