Your sister Tatum calls as you push someone else’s baby in a stroller. Your neighborhood in Philadelphia, it turns out, absorbs heat with its wide treeless sidewalks. Even in the early morning hours, you shirt collects a constellation of condensation off your back.

Before Tatum could say as much as hello, you yap about this spontaneous nanny job. A 13-month-old who loves animal noises and whose parents lived in New York and migrated to Philly to raise her, people from big families and creative b...