A four-year-old boy has died in hospital, days after his youngest siblings perished in a shed fire on a property in Victoria.

The boy and his three siblings were playing together in the rear shed of a Corio property near Geelong on Sunday morning when it caught alight.

Three-year-old Saige McGregor and 18-month-old Ashlynn McGregor died at the scene while the four-year-old, Isaac McGregor, and his six-year-old sister Mavis were flown to the Royal Children’s Hospital.

Victoria Police confirmed Isaac died in hospital on Wednesday afternoon, while his six-year-old sister remained in a critical condition.

All four children were inside the shed when a couch was set alight.

The children’s father Kane McGregor said on Tuesday doctors had postponed surgery for his son, fearing the boy would not survive it.

He revealed Isaac’s mother, Jasmine Beck, found him huddled over his two youngest siblings so they would not burn.

“He stayed there until he died himself. They revived him,” McGregor said.

“I couldn’t be any prouder of him.”

He said Isaac had sustained burns to more than 80 per cent of his body and was showing signs of liver and kidney failure.

A GoFundMe page for the children was updated on Wednesday to say Isaac “sadly grew his wings and reunited with his baby brother and sister”.

Donations to the fundraiser had surpassed $55,000 by Wednesday afternoon.

“[Isaac] will always be remembered as the heroic young boy who gave his life trying to protect his siblings,” GoFundMe organiser Daina Leech wrote.

“We thank everyone so much from the bottom of our hearts for all the donations and support.

“This community is absolutely incredible.”

A couch inside the rear shed, used as a dog bed, caught alight while the children were playing.

Police are not treating the fire as suspicious as arson and explosive detectives continue to investigate its cause.

McGregor has suggested police were probing whether the children were playing with a click lighter before the fire ignited.

A community vigil will be held for the children at Corio on Saturday.

Their mother, 26-year-old Beck, was inside the home at the time and ran to the shed, trying desperately to rescue them.

The children’s aunt and uncle, Sarah Guardiano and Anthony McGregore, said Beck ran into the shed after the couch caught fire.

“She had to move a couch that was on fire to pull the kids out,” Guardiano said on Monday.

“Jasmine did her very best.

“Pulling all those babies out, like you can imagine, you know, having to call the fire brigade, having to put out a fire, having to pull out your children. She did the best she could to get them all out as quickly as she did.”

McGregor said on Tuesday it had been the worst 36 hours of his life.

“Absolutely broken-hearted having to let go of my two youngest baby’s (sic). [The] only thing keeping me together is holding onto and fighting for my other two,” he wrote in an emotional social media post.

“You are both so strong and way to (sic) stubborn to give in. So keep fighting the winning fight as youse have this far and I’ll lift the world for youse.

“Couldn’t be any prouder of you two. Long road to recovery but [the] only way to go is up. Rip (rest in peace) wiggles and saigey. I love you two so much.”

-with AAP