latchkey

/L AE1 CH K IY0/
noun
  1. 1

    A key, especially to an outside door.

  2. 2

    A child who is given a key to the home and is expected to remain at home alone (without adult supervision until the parents return from work).

Synonyms

adjective
  1. 1

    Equipped with a key; generally in the phrase latchkey child.

Source: https://en.wiktionary.org/wiki/latchkey