Long before they were buffered by padded plastic and Astroturf, boys in 1911 in East Orange, N.J., climbed and dangled from bars more than 30 feet off the ground. Parents’ concern at the time? Socialization between classes and races. According to author Amalie Hofer Jerome, playgrounds were preferred places to challenge early twentieth century social norms. “On a recent visit to a certain playground in which are mingled many nationalities, including blacks and whites, I...