The family of a young Perth boy who died in a house fire last week say his organs have helped save the lives of four other people.

Four-year-old Thyreese Alaniz-Rowsell died after the blaze engulfed his Thornlie home, also killing his 69-year-old great grandfather Hugo Alaniz.

Thyreese’s uncle Raul Alaniz said the family took come comfort from the fact that his organs have been donated.

“His organs went to help someone make a better life for themselves,” he said.

“That’s something positive we can take out of this tragedy.”

Thyreese has helped save four lives. Picture: 9NEWS (9NEWS)

The close-knit family has been left devastated by the loss. Picture: 9NEWS (9news)

Fire investigators are trying to piece together how the blaze started, seizing a motorised toy from the property for examination.

“One of the things police are looking at is the bike my parents bought him for his birthday, an electric bike,” he said.

“They think that might be a part of what caused it but they don’t know.”

It has been confirmed the fire was accidental.

Mr Alaniz removed batteries from a smoke detector just a few days before the fatal fire.

Raul said the family is urging everyone to check their smoke alarms are working this winter.

Hugh Alaniz was killed in the blaze. Picture: 9NEWS (9news)

The close-knit family has been left devastated by the loss, and are rallying around Edith Alaniz - the wife of Hugo and Thyreese’s great-grandmother.

The home in Thornlie was not insured and very little was salvaged from the property.

Mrs Alaniz still has a mortgage on the property, which will be demolished.

“The house is very burned. Everything is gone,” he said.

The home in Thornlie was not insured and very little was salvaged from the property. Picture: 9NEWS (9news)

“We’ve taken mum through the house, she had to see that the house is gone.

“We’re just trying to get mum calm before the funeral. She’s lost her whole life.”

Friends of the Alaniz family have set up a fundraising page to help cover the cost of the funeral.