“Regular amounts of time each week in which the child is left alone or left with a sibling younger than age 13” (Cappizano, Tout, & Adams, 2000).
“A child between the ages of approximately 6 and 13 who spends time along or with a younger sibling on a periodic basis” (Cole & Rodman, 1987).

Cappizano, J., Tout, K. & Adams, G. (2000). Child care patterns of school-age children with employed mothers, Occasional paper number 41. Washington, DC: The Urban Institute: Cole, C., & Rodman, H. (1987). When school-age children care for themselves:  Issues for family life educator and parents.  Family Relations, 36, p.93.