A man has been charged with the murder of his five-year-old son after the boy was allegedly stabbed at a Sydney home.

The boy was allegedly attacked with a knife by a man living at the same address during a domestic argument in Paul Place, Carlingford.

The 36-year-old father is in custody at Castle Hill and will face Parramatta Bail Court today.

The five-year-old boy, who cannot be named for legal reasons, in the arms of his father. Picture: Facebook (Supplied)

The family, who live at Paul Place, Carlingford, on a night out. Picture: Facebook (Supplied)

The boy was taken to Westmead Children's Hospital but could not be saved. Picture: Facebook (Supplied)

In what has been described by police as a "heartbreaking incident that would affect the community", officers were called to Moseley Street at 7am.

The boy's grandmother had got him out of the house, cradling him into a car and driving away from the scene, before pulling over and calling triple-0.

He was treated by paramedics and taken under police escort to Westmead Children's Hospital, but could not be saved and died an hour after arriving, at 8.02am.

The shocked grandmother is helped into an ambulance. Picture: 9News (Nine)

The boy's grandmother got him out of the house during the domestic dispute. Picture: 9News (Nine)

The grandmother, 60, was treated for shock.

Superintendent Rob Critchlow, Hills Area commander, yesterday described the level of violence meted out to the boy as "horrendous".

"The level of violence is horrendous ... it's probably about as bad it gets, really," he said

The grandmother pulled into the street to call triple-0. Picture: 9News (Nine)

The scene in Moseley Street, Carlingford, this morning. Picture: 9News (Nine)

"To have a young child murdered in his own home brutally is as bad as it gets."

He confirmed the family was Australian and said the grandmother's actions had been "heroic and caring".

Police responded to calls at around 7am. Picture: Hannah Sinclair (Nine)

Supt Critchlow said a child being murdered in his own home was 'as bad as it gets'. Picture: 9News (Nine)

The child's mother went to hospital, but it's not known where she was at the time of the stabbing.

Supt Critchlow said the man in custody might be known to police and the weapon, believed to be a knife, had been secured.

As well as affecting a tight-knit community, he said the killing would have a "serious impact" on emergency service workers.

Witnesses have spoken of their shock.

(9news)