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Valmai Horlor rushed to the upper storey of her home and fought against a wall of flame to try to rescue the young boy in her care.
But she was helpless against the fire that rapidly engulfed the house and was forced to retreat while firefighters entered through an upper storey window to reach trapped 8-year-old Aaron John Harwood.
The boy was pulled clear, but despite efforts of ambulance staff to resuscitate him, Aaron died in Christchurch Hospital on Sunday evening.
Mrs Horlor suffered minor burns, singed hair, blistering and "things consistent with being attacked by a wall of flame", police said.
Police said smoke alarms alerted the occupants of the Christchurch home, but without an exit plan, there was nothing more that could have been done for Aaron.
They are refusing to release their findings on what started the fire, but have ruled out fireworks and are not looking for anyone in relation to it.
"The fire demonstrates two things," said Detective Sergeant Ken Legat, a specialist fire investigator for police.
"One, the importance of having working smoke alarms ... but you also have to have an exit plan."
Aaron was in the custody of the Open Home Foundation after being taken from the custody of his immediate family.
