The three-year-old is understood to have been playing in the garden of a house when the dog attacked him, causing facial injuries.
The incident happened at around 8pm on Tuesday in the Bellfield area of Kilmarnock.
Paramedics rushed to the semi-detached house in Campsie Road and took the youngster to Crosshouse hospital.
He had surgery for facial injuries and is understood still to be recovering in the facility.
It is understood the dog was owned by the boyfriend of the three-year-old's mother and the attack happened in the dog owner's mother's home.
The pit bull was destroyed with the owner's consent after it attacked the child.
The family were too upset to speak following the incident, but locals spoke of their shock and concern.
One dog owner said: "Any dog has the capacity to bite anyone but it's absolutely terrible about the wee boy.
"I didn't even know anyone had pit bulls in this area, I thought they were banned."
A neighbour said the tragedy was "absolutely shocking" and added that "it was just a horrible, horrible accident".
Another neighbour living near the scene of the attack added that the family were "devastated".
He said: "There have never been any problems with the dog, it was always fine as far as I could tell."
A local man said he had sometimes seen the dog in the street, but it was friendly and had never been known to be aggressive in the past.
Police also visited the quiet road on Tuesday night and investigated the scene.
A 25-year-old man is the subject of a report to the procurator fiscal in relation to the Dangerous Dogs Act.
The Government is considering measures to improve the safety around dogs which are known to be dangerous after a spate of attacks in recent years.
Microchipping and licensing are two of the suggestions which have been discussed by the authorities, as well as forcing the animals to wear muzzles in public and ownership suitability tests.
The First Minister met last year with the mothers of three child victims who had been attacked by dangerous dogs before launching a public consultation following the meeting.
The summit on dangerous dogs was held in March this year, and was attended by Justice Secretary Kenny MacAskill and Rural Affairs Secretary Richard Lochhead.
They met with representatives from groups including Police Scotland, the Dogs Trust, Guide Dogs Scotland, the Kennel Club, local authorities and the SSPCA to discuss options on improving safety.