latchkey (n.) Look up latchkey at Dictionary.com
also latch-key, 1825, a key to draw back the latch of a door, from latch (n.) + key (n.1). Latchkey child first recorded 1944, American English, in reference to children who come home from school while both parents are at work.
Many elementary school principals and teachers have always known the "latchkey" child or the "eight-hour orphan." [New York State Teachers Association, "New York State Education," 1944]