Story, first published: Galaxy Science Fiction, April 1958
The perfect babysitters of "The Sitters" ... are benign aliens who transfer the gift of youth to the older citizens of Millville by literally stealing it from the young.
Ewald, Robert J.: When the Fires Burn High and the Wind is from the North, p.53
Small town sf about aliens living among humans, and trying to impove humanity by saving the essense of childhood. Generally cozy and nice.