latchkey child

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A usually school-age child who regularly spends part of the day unsupervised at home while the parents are at work.

Wiktionary

1. n. latchkey kid

GNU Webster's 1913

1. 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.

WordNet 3.0

1. n. 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)