Susan* bought her 6-year-old son John an iPad when he was in first grade. “I thought, ‘Why not let him get a jump on things?’ ” she told me during a therapy session. John’s school had begun using the devices with younger and younger grades — and his technology teacher had raved about their educational benefits — so Susan wanted to do what was best for her sandy-haired boy who loved reading and playing baseball.

She started letting John play different educational games on his iPad...