Neighbours told local television network, WATE 6 the boy shot his neighbour after the pair had argued over the girl's puppy.
Detectives say the boy used his father's 12-gauge shotgun to shoot McKayla Dyer through a window as she played in her yard over the weekend.
A neighbour told WATE 6 that before shooting McKayla, the boy had asked if he could see her puppy.
The victim reportedly refused, and soon afterwards, the boy shot her.
McKayla's uncle, Dallas Peterson says the family is struggling to accept her death.
"I was in shock, I just couldn't believe it because I know she was such a sweet little girl."
It comes just days after a fatal shooting of nine people at a college shooting in Oregon in a deadly rampage at Umpqua Community college.
"This is not a gun issue, this is a parenting issue. Our problem with our country now is that we're not stepping in and being good parents. We're not stepping in and teaching them right from wrong," Mr Peterson said.
The boy, who has not been named, appeared in court overnight.