Child dead, two left critical in 'traumatic' stabbing

Investigations are continuing after a young child was left dead and two others fighting for life following a stabbing in a small Blue Mountains township.

Members of the public spotted the three children and a woman with stab wounds inside a vehicle on Waratah Road at Valley Heights shortly before 1pm on Sunday.

After finding one child dead, the witnesses rushed to help the survivors, delivering first aid while waiting for emergency services to arrive.

Paramedics treated all three at the scene before taking them by road to Westmead Hospital and Westmead Children's Hospital in critical condition.

Hours after the incident, NSW Police Superintendent John Nelson said the father of all three children had been notified, but not yet the wider family.

The father had been "going about (his) business" at work when he received the call from police, he said.

"You can imagine receiving that tragic news," Supt Nelson told reporters.

He said the woman had not previously come to the attention of police and that investigators were not currently looking for anyone further in relation to the incident.

She was thought to be aged in her 40s and the children all under five years old.

Supt Nelson praised the efforts of the "upstanding" members of the public who responded to the tragic scene.

"People are just going about a lovely Sunday afternoon and being confronted with this is traumatic and tragic for anybody," he said.

Responding officers would not be spared the hardship and would have access to available supports.

"Like any human being, it's a tragic circumstance to be exposed to," Supt Nelson said.

"They're highly professional officers, and they responded in such a professional way."