When David Hiller’s two daughters checked into Camp Echo, a bucolic sleep-away camp in Upstate New York, they relinquished their cellphones for seven idyllic weeks away from their digital lives. But not Hiller: His phone rings 10 times a day with notifications from the summer camp’s facial-recognition service, which alerts him whenever one of his girls is photographed enjoying their newfound independence, going water-skiing or making a new friend. His daughters don’t really know about the facial-recognition part, he said.