Gwent Police said the dog was destroyed by firearms officers, adding that no other animals were involved in the attack.
Officers were called to an address in Pentwyn, Penyrhoel, at around 3.55pm on Monday. Eyewitnesses told WalesOnline there were up to 12 police cars and four ambulances at the scene.
One neighbour described their shock, saying: “I’ve never seen anything like it before in my life. It was horrific.”
A nearby resident told PA: “The dog has been a nuisance on the estate for a bit of a time. I took our son down to the local shop a few days ago, and he was down there then and he was lunging at my son.
“My son is three years old and I had to pick up my son just to move around the dog because of the size on him. He was absolutely huge.
“He wasn't a pet - he was taking the owner for a walk, you know. He was massive.”
Local councillor Geoffrey Whittle said he had been speaking with devastated staff at the child’s school and had asked the council for support for staff and pupils.
Wayne David, MP for Caerphilly, said he “extremely saddened” by the incident, adding that “lessons must be learned”.
“It must be appalling for the family,” he told WalesOnline.
“It’s important that the community pulls together and works closely with police.”
Chief Superintendent Mark Hobrough said: “Officers will be making further enquiries at this time and will remain at the scene as the investigation progresses.
“It is possible that you may see ongoing police activity in Caerphilly as part of this work.
“You may have also seen an increased presence earlier today while officers were attending the incident, but please do not be alarmed.
“If you have concerns or information then please do stop and talk with us.”
Officers urged anyone with information to call 101, quoting log reference 2100392510.
Press Association contributed to this report
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