Neighbours, alerted to the blaze by screams, fought to save the family but had difficulty breaking through double-glazed windows.
Police later identified the victims as Anne-Marie Kemp, her son, David, five, and daughters Jamie-Lee, four, and Carrie-Ann, two. The father, Gary Kemp, suffered serious burns.
The blaze gutted the upstairs of their three-bedroom semi- detached home in Sittingbourne, Kent.
Carrie-Ann was taken out alive from the building but died from her injuries. Mr Kemp, 25, was taken to the specialist burns unit at East Grinstead hospital, West Sussex, where he was said to be in a "critical condition".
A neighbour, Michael Lock, said: "There were about 20 people all trying to smash the windows but they had double glazing and they could not get at them." Another neighbour, Steve Martin, said: "I heard the kids screaming inside and so I grabbed a ladder and dashed back to the house. I put the ladder up but by the time I got to the top there was silence - I can only assume the children were either unconscious or were already dead.
"I smashed the window but I couldn't see anything inside. The smoke was too thick to get past and I had to give up."
A local resident, Ivan Bowden, said of Mr Kemp: "The poor guy was sitting on his lawn. He was terribly burned and he was holding his head in his hands, crying over and over again, 'My babies, my babies'."
It was thought he tried to save his family before fleeing from the flames. The family dog, a labrador puppy, was unhurt.
Kent divisional fire officer, Paul Trewin, said: "The fire started in one of the bedrooms and spread very quickly. We are still trying to find the exact cause of the fire but it is looking like an accident rather than suspicious."Reuse content