"The day after that conversation, my 12-year-old daughter took her toddler brother to a playground, only to return very soon and very shaken. This park is Harlem in miniature. There are two playgrounds, less than 300 yards apart, an old basic one and a new shiny one. In the new playground, well-dressed toddlers of different races, most of them minded by dark-skinned nannies, played together. At the old one, a few black kids played while their older siblings kept reluctant watch. A couple of drunken men loitered nearby. Apparently, one of them had lashed out at my daughter, calling her...