The Nissan Pulsar was heading west on State Highway 83 when it crossed the centre line and left the road shortly after 3.30pm.
Constable Nayland Smith, of Omarama, said police were disappointed the child had not been travelling in a child restraint.
Driver fatigue was a suspected cause of the crash, he said.
Const Smith said the woman might have suffered fracture injuries while the boy, who was in the front seat, appeared relatively unharmed.