The fatal crash happened near Chichester in West Sussex yesterday at 4pm, when a family car collided into another vehicle.
A seven-month-old girl in the family's Mazda 3 car was killed, while a boy, aged 5, is fighting for life after being airlifted to hospital.
A 27-year-old woman passenger in the car suffered serious leg injuries, while the 31-year-old driver suffered minor injuries.
All the occupants are from the West Sussex village of Pulborough.
The driver of the Vauxhall Corsa which collided with the Mazda on the B2133 at Newpound Common, near Wisborough Green, suffered minor injuries.
A Sussex Police spokesman confirmed this morning: "The seven-month-old girl was taken to Worthing Hospital, where she sadly died at 6.36pm.
"The five-year-old boy was airlifted to St George's Hospital at Tooting in a critical condition.
"Both were passengers in the Mazda. A 27-year-old woman passenger suffered serious leg injuries and was also taken to hospital in London.
"The 31-year-old driver suffered minor injuries. All of the occupants are from Pulborough.
"The driver of the Corsa, a 36-year-old man from Pulborough, sustained serious chest injuries and was taken to Worthing Hospital."
The road was closed for six hours while police examined and cleared the crash scene, before being reopened at 10pm.
The force spokesman added: "Police are appealing for witnesses or anyone who may have seen the vehicles in the area shortly beforehand to report online or call 101 quoting Operation Gilford."
