Perth house fire kills boy, great-grandad

A four-year-old boy has died almost 24 hours after a Perth house fire that also killed his great-grandfather.

The blaze on Rushbrook Way, Thornlie, was reported about 6.15pm on Wednesday and firefighters rescued the pair from a bedroom.

Hugo Alaniz, 69, was pronounced dead on arrival at Fiona Stanley Hospital.

His great-grandson Thyreese was also sent to FSH before being transferred to Perth Children's Hospital.

Police confirmed the child's death about 5pm Thursday, but family had already told reporters arrangements were being made to donate his organs.

"He's going to live through someone else," Raul Alaniz told the Seven Network on Thursday.

Inspector Dean Snashall told reporters at the scene the pair was unresponsive when police and firefighters arrived.

Neighbour Darren Broadbelt said he saw the blaze engulf his friend's home.

"It was pretty bad coming through the window - it popped a roller shutter off the front," he told the Seven Network.

He said he had lived opposite his neighbours for many years.

"I've been over their house, broke bread with them, they're lovely people, it's a tragedy," Mr Broadbelt said.

The arson squad is investigating, with the cause of the fire yet to be determined.

Police will prepare a report for the coroner.