Fire and Emergency New Zealand assistant area commander Mike Bowden told the Herald it is believed four children and their father were in the house when the fire started.
Bowden said Arianna was in the room where the fire was most intense, and her brother was found in the doorway outside this room.
More than $122,000 had been donated to support the grieving family as of 7pm Thursday.
The Givealittle page was set up on Wednesday by Brayden's aunty Ashley Looyer.
"Your donations will help the family of Brayden and Arianna cover funeral costs and help them get back on their feet without the financial burden that will accompany this horrific loss," the page reads.
Many people expressed their condolences on the page, leaving messages of sympathy and love.
One donor said: "No words can express the sadness I feel for you."
Another donor said: "Fly high angels."
In an interview with the Press in Christchurch, Brayden's grandmother, Angela Martin, said the children's mother, Nicole Mulligan, had called her in an hysterical state soon after the fire in Vivian St, Burwood on Tuesday night.
Last night, a devastating house fire ripped through the house of a Christchurch family. Photo/Supplied
"She said to me, 'my babies have died, I'm having a nightmare'."
The children's mother believed a fan heater was to blame for the fire, Martin said.
It had been recently bought to keep baby Arianna's room warm, she said.
Mullilgan's partner Des Cooke, the father of Arianna, had rescued two of the four children in the house but flames and smoke forced him and a neighbour to retreat as they tried to save the other two.
"Des heroically saved the lives of his two other young children and suffered injuries himself after re-entering the burning house in an attempt to save Brayden and Arianna," says a Givealittle page set up for the family.
NZ Gift of Love and Strength a Christchurch-based charity is taking donations for the family, which can be dropped of at the local Little Explorers Preschool.
