The car driver has been arrested on suspicion of causing death by dangerous driving following the crash near the village of Horton, Dorset, on Sunday.
Officers were called to reports of a "serious road traffic collision" on the B3078 at 21:20 BST.
A blue MG car carrying five people had left the road and collided with a wall before overturning, Dorset Police said.
The girl's next of kin have been informed and the coroner notified, police said.
An eight-year-old boy from Wimborne suffered potentially life-threatening injuries and was taken to hospital in Southampton.
An 11-year-old girl from Corfe Mullen suffered bruising and was taken to Poole Hospital for treatment
The driver of the car, a 28-year-old man from the Wimborne area, has been arrested and is currently assisting police with their enquiries.
He and a female passenger, 28, were treated for minor injuries and shock.
Police have appealed for information from anyone who saw the car prior to the crash.
The road, between Witchampton and Horton, has reopened after investigations of the scene finished.
A two-year-old girl has died and two other children were injured after car hit a wall and landed on its roof.