Quebec provincial police said the fire on Lacroix Street was called in just before 5am.
The family’s two-storey stone house was found ablaze when firefighters, paramedics and police arrived at the scene. Firefighters continued to battle the fire for more than three hours later from outside.
A man and woman lived in the home, with their five children. A family friend, a 12-year girl, was sleeping over for the night.
One girl, 12, was rushed to hospital then transferred to the Children's Hospital of Eastern Ontario in Ottawa. She and her father are in critical condition. The mother and two children remain in hospital.
Neighbours were woken by screams for help. They ran outside to find the home engulfed in flames. The parents were able to get four children out - but two were trapped in an upstairs bedroom.
In all, four children and two adults were taken to hospital, police said, the parents, three of their children and the family friend.
Two other children were at first unaccounted for, police said, and firefighters began to search the home at about 9 a.m. The bodies of the two children were found soon after in the home's second floor.
The people sent to hospital suffered various injuries, including cuts and burns.
Father tried to re-enter home to rescue kids
Neighbour, Sylvain Leblanc, said he woke up at about 4:30 a.m. to the sound of a woman, the mother, running outside screaming as flames came from the home.
Leblanc said the father tried to enter the second floor of the house with a ladder to reach his children, who were still in the burning house.
Leblanc then said three children were in "bad, bad shape" on the sidewalk. He and his wife helped them inside their home to give them comfort, he said, before paramedics performed some initial treatment on the children in his kitchen.
"One of the kids was burned on the hand, on the feet, really bad. I have blood in my kitchen. I have a lot of paramedic stuff. It's like a war zone. That's my kitchen now," he said.
"I think when I'm going to be alone, I might cry."
Leblanc also told the CBC's Ryan Gibson the family, which included the parents and five children, moved into the home about one year ago.
The cause of the fire wasn't immediately known. The building also remained unstable, which made the investigation more difficult for fire investigators.
Officials from Maria-Goretti school in Thurso, where some of the children attended, and the local board of education issued a statement Wednesday saying counselling would be available for students.
Fire officials do not yet know if there were any working smoke detectors in the home. All day long, a steady stream of family members, friends and neighbours came by to share their grief.Suggest a correction