Police have charged a man with murder after he allegedly fatally stabbed his five-year-old son in their Carlingford home on Friday morning.

The boy died in hospital after his father allegedly stabbed him during a domestic dispute at Carlingford, in Sydney's north-west, described by police as a "horrible, brutal crime".

Police said the boy lived with his father and grandmother at the house in Carlingford.

After the boy was stabbed, the grandmother, aged in her 60s, took him to safety in a nearby street and called triple-0, 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."

Police said the boy was stabbed with a knife which has been located but would not disclose the extent of the child's injuries.

A woman covered in a white sheet was led by police to an ambulance. ( ABC )

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.

Natalie and Hank Lewin lived next door said the boy and his parents had lived at the home for about six months.

Neighbours said the boy and his father had a good relationship. ( Facebook )

They described the boy as "very sweet and happy" and the relationship between father and son as "doting".

"It's absolutely shocking, [the father] and his boy were close," Mr Lewin said.

"He cared for that boy, he was very patient with that boy and held him and nursed him like you would your son."

Mrs Lewin said the boy played with her two children and she recently minded him for a few hours.

"I'm still shaking because … I don't even know how to tell my kids," she said.

Neighbours have described the boy as "sweet and happy". ( Facebook )

The couple said the father was a mechanic but had not worked for a month as he had been unwell and spent some time in hospital.

"I think they were going through tough times because it was only her working," Mrs Lewin said.

Mr Lewin said the father was "an easygoing Aussie bloke" and they would occasionally share beers and talk about cars and the footy. However in the past month he had stopped speaking to them and would only wave hello.

Mrs Lewin said early on Friday morning she heard noise that sounded like someone knocking on a door but went back to sleep. When she awoke, police and emergency services had arrived.

James Wynne, 84, lives across the street from the house and was shocked to hear the news.

"I'm quite upset about it. I don't want to see children hurt at anytime like that, particularly a five-year-old — he's only a baby," he said.

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

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.

The 36-year-old man, who is known to police, was charged at Castle Hill Police Station and refused bail to appear before the Parramatta Bail Court on Saturday.