noun
latchkey child; latchkey kid
Refers to a child who carries a house key and returns home alone because parents are working. Common in discussions of child welfare and social change.
共働きの家庭が増えて、鍵っ子も多くなった。
With more dual-income households, the number of latchkey children has increased.
小学生のとき、私は鍵っ子だった。
When I was in elementary school, I was a latchkey kid.
留守番 means 'looking after the house while others are away,' whereas 鍵っ子 specifically refers to a child who does this alone with a key.
Compound of 鍵 (key) and 子 (child), with a geminate consonant inserted for euphony. The term became common in Japan during the 1970s and 1980s as more mothers entered the workforce.