Lancaster County authorities say it happened around 1:10 a.m. Friday in Rothsville.
The Intelligencer Journal of Lancaster reports Warwick Township police say Milee Beiler left the home with her brother as their father slept.
The family says the boy told them he had taken his sister to look at some goldfish.
Authorities say the boy went to a neighbor's house looking for help. Police responded and found the girl face-down in the pond.
The Lancaster County coroner has ruled that the girl drowned and the death was an accident.
Information from: Intelligencer Journal/Lancaster New Era , http://lancasteronline.com