Kentucky State Police said the toddler was shot just after 1 p.m. CDT Tuesday in Cumberland County and was taken to a nearby hospital, where she was later pronounced dead.
Cumberland County Coroner Gary White told the Lexington Herald-Leader that the children's mother was at home at the time.
White told the newspaper that the boy received the rifle made for youths last year and is used to shooting it. He said the gun was kept in a corner and the family didn't realize a shell was left inside it.
White said the shooting will be ruled accidental.
An autopsy is scheduled for Wednesday.
Copyright 2013 The Associated Press.