The three children and a woman were found with stab wounds inside a vehicle on Waratah Road at Valley Heights, a small township in the NSW Blue Mountains, on Sunday.
Members of the public alerted authorities about 12.45pm after finding one child dead while delivering first aid to the woman and surviving children.
NSW Police Superintendent John Nelson said the father of the three children had been told about the attack while "going about (his) business" at work.
"You can imagine receiving that tragic news," he said late on Sunday afternoon.
The woman was thought to be aged in her 40s and the children all under five years old.
She had not previously come to the attention of police, Supt Nelson said, but investigators were not currently looking for anyone further in relation to the attack.
The woman and two surviving children were undergoing surgery at Westmead Hospital and Westmead Children's Hospital, respectively.
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 support mechanisms.
"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."
The weapon used and the circumstances surrounding the incident would be a matter for investigators, Supt Nelson added.
A crime scene has been established with local police and homicide squad investigators at the location.
Blue Mountains councillor Darren Rodrigo described the news as "absolutely horrendous and tragic".
"Our deepest love and condolences to the immediate family and our prayers with all the children involved," he said.
"A massive thank you to the community members (who) intervened to provide medical assistance and to our first responders for their incredible support and courage during this time."
Anyone with information about the stabbing or who had footage of the attack is urged to contact police.
Lifeline 13 11 14
beyondblue 1300 22 4636
1800 RESPECT (1800 737 732)
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