latchkey child 
noun
A child who is at home without adult supervision for some part of the day, especially after school until a parent returns from work.As a child, Ivey, 42, was a latchkey kid until junior high school, when her stepmother retired....
- Dec is the baby of a large Irish Catholic brood of seven children while Ant is the eldest of three, a latchkey kid who made his own lunch and who, after his parents divorced, lived with his mum and looked after his younger sister.
- Now, when I was a kid, my mom worked, and I was a latchkey kid.