The Collaborative International Dictionary of English v.0.48:

latchkey child \latch"key` child\, n.
   A child who frequently spends part of the day at home without
   adult supervision, especially a school child who returns home
   before the parent(s) return from work. Called also door-key
   child.
   [PJC]

WordNet (r) 3.0 (2006):

latchkey child
    n 1: a school-age child who is home without adult supervision
         for part of the day (especially after school until a parent
         returns home from work)