AUG. 26, 2017 — Parents of preteens know the dismay that comes with this image: kids flopped on the couch for stretches of time, silent and mesmerized by smartphones. Maybe it’s Snapchat, maybe its Facebook or Instagram — but they scroll and scroll, oblivious to an outside world that beckons with all things tangible and sunny: a pool, a baseball diamond, a bike ride. The things that lure kids into their own walled-off worlds have evolved over the years: Pac-Man in the ’80s, Nintendo in the ’90s.