latchkey child

from The American Heritage® Dictionary of the English Language, 5th Edition.

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

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun latchkey kid

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun 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)