Three-week old baby Reggie was attacked by the family dog, a Patterdale terrier, and he died in hospital as doctors could not save him.
It was gathered that the mother of the deceased baby was at a family event following a bereavement, a family member said.
Their family dog, called Tricky, is being kept in kennels and is expected to be killed.
Northumbria Police said a 30-year-old man had been arrested in connection with the incident.
“We are carrying out enquiries to establish the circumstances.”
A family member, who did not want be identified, said the baby’s mother was not in the house at the time of the attack, and added that she never left her son alone with the dog.
Neighbours yesterday spoke of their shock at the death of Reggie, who is believed to have four brothers and sisters.
One, who did not wish to be named, said: “I know terriers very well. I think the dog had become jealous of the baby and that is why this has happened.
“It was Ryan’s dog as far as I’m aware. He is a lovely lad, hard-working and an excellent father. He will be devastated.”
Criminal psychologist Paul Tweddle, 50, who lives opposite the family in Sunderland, Tyne and Wear, said: “They are a caring couple in their 30s and are a lovely family. This is a complete tragedy.
“I see the terrier from time to time. Sometimes I see it clambering at the fence and it gets out so I shout to Ryan and he comes and gets it.
“It is a tiny dog and I cannot say whether or not it is dangerous. Sometimes appearances can be deceptive.
“I am so shocked. It should be such a happy time with a new baby. I have children myself, so you can imagine how I am feeling.”
Another neighbour, Denise Haley, 59, is grandmother to a six-week-old baby called TJ.
She said: “Maria was in the house a few weeks ago with Redgie. She is a lovely girl and I simply do not know how she will cope with this tragedy.
“I am scared of any dog but that dog seemed okay. It is a tiny dog and brown in colour.
“I am in total shock – me and my daughter have been really upset about it. We grabbed TJ straight away and gave him a huge hug.”
Source: UK Mirror.