Police have hailed the "brave" man who intervened in the alleged stabbing of two children on a motorway, north-west of Adelaide.

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Two boys, aged eight and three, remain in hospital after allegedly being stabbed by their mother on the North-South Motorway last night. 

SA Police have alleged the 35-year-old Modbury Heights woman stopped her car and removed the two children before attacking them. 

A man was able to disarm the woman, and members of the public helped detain her before calling police. 

Superintendent Selena Dinning has praised the strangers who stopped and called authorities, including a man who intervened soon after the incident.

“We are extremely grateful to the people that stopped and rendered assistance, intervening in this incident, as no doubt the situation would have been much worse had they not stopped," Superintendent Dinning said. 

“If this man hadn't stopped, I'm sure the situation would have been much worse.

"We're very grateful that he stopped, we're also very grateful [to] the two other members of the public that assisted him.”

Superintendent Dinning said the woman was driving the family car. 

"We don't know what triggered the episode, we don't know at this point in time where she was going or what she was doing," she said.

"From my personal perspective, the expressway incident is the first that I became aware of this particular family.

"But all of that history will be brought forward before the court to consider."

The two children were taken to the Women's and Children's Hospital in a critical condition.

Superintendent Dinning said one child had undergone surgery, and the other was about to have surgery following the incident.

Both children are in a stable condition.

The mother is currently at the Royal Adelaide Hospital and is expected to face a bedside court hearing this afternoon over the matter.

Premier calls describes incident as 'absolute tragedy'

SA Premier Peter Malinauskas said the alleged attack was "almost impossible to comprehend". 

"If the allegations are proven to be true, God knows what circumstances have led to this absolute tragedy," he said.

"I don't want to make assumptions, but presumably, when we see a tragedy of this nature I think it's a very real possibility that this is a family that has interactions with government agencies in the past but [I] don't know that at this stage and not something I'd be able to comment on without appropriate briefing on it."