You may not notice it right away, but an unsettling quiet settles over a neighborhood when children disappear. Playgrounds feel empty, fewer strollers pass by, and fewer parents shout “stop” at little ones scooting along the sidewalk. The local school begins to shrink, changing who encounters whom daily, who comes to recognize neighborhood, and how often residents are reminded that they are part of a community. The number of children in New York has been declining.