A child (usually younger like age 7-13) who comes home (usually from school) to an empty house and is left unsupervised and to fend for themselves until someone older comes home to watch over them. This is usually done for 2-4 hours. This name/phrase originates from the latchkey of a door because the child is often given the responsibility/privilege/burden of carrying a key to the house around with them (even in school). The key is typically worn around the neck or kept in a hidden area of the book bag but is also sometimes hidden under a door mat of the house, or in some other hidden location near the door.
See also: child neglect
Every day little Stephen has to come home to an empty house and raise himself from 3-6. He bears the burden of being a Latchkey Kid.