It happened around 2:40 p.m. Monday in the outer lanes of the Boulevard between Woodward and Strahle streets.
The witness said a green Toyota hit the woman and 4-year-old child as they used the sidewalk to cross a bridge.
The witness said it began when a rental van, possible a Ryder truck, was swerving in traffic and the Toyota swerved to avoid the truck.
That's when the car hit the mother and child, the witness said.
The mother was pronounced dead at the scene.
The child was taken to Saint Christopher's Hospital for Children and was undergoing surgery for head injuries.
The Toyota was still on the scene, but there was no sign of the rental truck.
Chopper 6 HD was over the scene as rescuers tried to revive her, with no success.
There was some concern that a second child was involved, but a search did not turn up any other victims.
The outer lanes of the Boulevard were shut down between Woodward and Strahle while the investigation continued.