People outside the no-smartphone group will develop specific ways of interacting, which you only "get" when you participate

Stuff you can't even know you're missing out, because you don't know what you don't know

Maybe calling someone without a heads up is considered rude, or ending sentences with a period is considered passive aggressive, maybe responding to someone right away is considered desperate or using certain words is okay IRL but not online

I agree that kids having smartphones is not good, but isolating an individual or small group from ostensibly the rest of the world in inconsistent ways that may interact in even worse ways is something you should be careful with, you might just be marking them for life