My 9-week-old daughter was lying, docile and stinky, in the crook of my arm on a flight from Hartford to Atlanta. I had prepared, like a warrior for battle, for this and every other possible complication. Change of clothes in case of accident: check. Natural wipes approved of by parents in my crunchy, overly child-centered town of Northampton, Mass.: check. Disposable diapers quietly judged by parents in my crunchy, overly child-centered town: check. I was flying alone. “I can do this,” I told myself, as I moved to the bathroom through an obstacle course of elbows and dozing heads. “I am a...