A five-year-old boy has died after being stabbed during a domestic dispute at Carlingford, in Sydney's north-west, described by police as a "horrible, brutal crime".

A man, 36, related to the child, has been arrested and police are expected to lay charges.

After the boy was stabbed, the grandmother, aged in her 60s, took him to safety in a nearby street and called triple zero, police said.

Police were called to the scene around 7.00am and the boy was taken to The Children's Hospital at Westmead with serious injuries but died shortly after.

"The level of violence is horrendous," Superintendent Rob Critchlow said.

"It's been a horrible and brutal crime against a very young, defenceless boy.

"It's probably about as bad as it gets, really."

© ABC News/Cassandra Bedwell A crime scene has been established and some surrounding streets have been cordoned off. Police said the boy was stabbed with a knife which has been located but would not disclose the extent of the child's injuries.

The child's grandmother was treated by the NSW Ambulance for shock at a nearby church but was physically uninjured.

Superintendent Critchlow said there was nothing more she could have done to save the child.

"She's behaved in a heroic and caring manner, as you would imagine a grandmother would.

"She's been presented with something terrible, and done her best to get the young boy to safety and to get him treatment.

"We're grateful to her for what she did do at the scene."

The death is "heartbreaking", Superintendent Critchlow said, and he is certain the community of Carlingford will "suffer very badly from this".

A resident of the street said the boy was a "sweet little guy" and "just a happy little kid" who had lots of energy.

The boy's mother, who was not in the house at the time, is now at The Children's Hospital.

© ABC News A woman covered in a white sheet was led by police to an ambulance. At the scene, police cordoned off the street and were seen leading a woman covered in a white sheet around the block to an ambulance.

A second crime scene has been established at a home on Paul Place.

The 36-year-old man, who is known to police, is currently speaking with investigators at Castle Hill Police Station.