The woman, who was believed to be in her 30s, was ejected from the vehicle and was confirmed dead at the scene, said ER24 spokesman Russel Meiring.
The baby girl was in a critical condition and given advanced life support by paramedics.
Two other children, aged 10 and 14, and a man sustained moderate injuries.
They were treated on the scene and taken to a nearby hospital for further medical care.
“While on route, paramedics had to begin CPR on the three-month-old patient to maintain her signs of life,” said Meiring.