Jack Lis died at a house in Pentwyn, Penyrheol, Caerphilly, on Monday afternoon.
The dog was destroyed by officers.
In a Facebook post, his mum Emma Whitfield wrote: “With so much heart break and before close friends and family see his name in the news, I have to announce our beautiful boy Jack was taken so tragically yesterday.
She said it was “not our dog, nor did it happen at our family home.
“He was out to play. We love you so much our sweet, sweet boy.
Jack was a pupil at Cwm Ifor Primary School whose head teacher, Gareth Rees, said the whole school community “has been rocked by the tragic events of the past 24 hours”.
Ch Supt Mark Hobrough, of Gwent Police, confirmed the attack “did not happen in the house owned by Jack’s family, but inside another property on a nearby street.
Paramedics went to the scene where the boy was pronounced dead.
“My condolences and thoughts are with Jack’s family, friends, school friends and everyone affected by this within the community,” Ch Supt Hobrough said.
“Officers are continuing to make further inquiries at this time and will remain at the scene as the investigation progresses.”
Neighbours described being alerted to “screaming” in the street on Monday afternoon.
Resident Lorrine Urvine said: “The kids – they were hysterical.
“And within minutes I’ve never seen so many police cars in my life.”
Neighbour Tracey Lawrence said: “The little boy that lives there ran over to a neighbour saying the dog was attacking his friend and the neighbour couldn’t help and couldn’t save the boy.”
Head teacher Mr Rees described Jack as a “bright and caring pupil who was popular amongst his peers and the teaching staff”.
“Our thoughts are very much with his family at this difficult time and we now need time and space to heal together as a school community.”
He said support was being made available for pupils and staff over the coming days “as we come to terms with this devastating news”.
