Police picked him up at 10 for walking home alone. At 21, he says free-range parenting gave him freedom. ["The incident led to a six-month investigation into potential child neglect by Child Protective Services"]

Bundled in heavy coats, hats, scarves, and mittens, Rafi Meitiv and his little sister Dvora had just spent a sunny Saturday afternoon in December 2014 playing at a local playground in suburban Maryland. After an hour, the 10- and 6-year-olds began their trek home, about three-quarters of a mile. While it was their first time walking from that particular park to their house alone, they'd traced the same route in Silver Spring dozens of times with their parents and had also ventured off alone to other playgrounds in the area before. About halfway through their journey, they were stopped by...