The 35-year-old woman has been charged with two counts of attempted murder after a bedside court hearing at at Royal Adelaide Hospital this afternoon.

Police allege the woman stopped her silver Honda sedan on the North South Motorway at Wingfield before 11.30pm and took her two young children, an eight-year-old boy and a three-year-old boy, out of the car then stabbed them with a knife.

A man who was driving past stopped on the motorway and intervened.

Other members of the public then also stopped to help and restrained the woman until police arrived.

"We think he was very brave in doing this and we are extremely glad he was in the right spot at the right time," Superintendent Selena Dinning said.

The two boys were taken to the Women's and Children's Hospital and treated for critical injuries.

Dinning said both boys suffered a number of stab wounds.

They are still in hospital after undergoing surgery in a stable condition.

Dinning described the incident as "horrific".