Sometimes, and perhaps increasingly, the pleasure of movies lies in the small things. The congenial Netflix romantic comedy A Family Affair, written by newcomer Carrie Solomon and directed by Richard LaGravenese, a veteran director and screenwriter who knows what he’s doing, mines some territory that’s already familiar from a movie released earlier this year: An “older” single mother tumbles into an affair with a much younger man, inviting judgment and ridicule from those around her. We just saw that idea...