The boys had told their parents that each was staying at the other's house on the Roundhills Estate in Waltham Abbey, Essex. Instead, they slept on a old mattress in a garage used for storage.
The children used candles to light the garage beneath three flats. The fire broke out after two of the children left to do a newspaper round.
Thomas Flynn, the boy who died, is thought to have been overcome by smoke as he slept. It was not until firemen had put out the fire that his body was discovered. His two 12-year-old friends were later being comforted by their families. An 84-year-old woman who lived above the garage was treated for smoke inhalation.
Police said an inquest would be held into the boy's death. Essex fire brigade said the garage was used by the parents of one of the boys to store furniture. Thomas's body was found as firefighters were turning over and damping down remains at the back of the garage.
Scotland Yard said a post mortem examination would be held today at St George's hospital, in Ilford, Essex.
The dead boy's family later spoke of their grief at the "stupid accident" which robbed of them of the football-loving boy, who had three sisters, Rosie, 18, Michelle, 17, and Joni, 15.
A distraught Michelle said: "It was such a stupid accident. It was nobody's fault, just one of those things. It was just a normal sleep-out, just a camp-out in a friend's garage."
Other relatives described Thomas, who attended the King Harold school in Waltham Abbey, as a fun-loving child who liked nothing better than kicking a football around with friends. His dream was to play for his favourite team, Spurs.
Tommy's mother Barbara, 38, a teacher, was too grief stricken to speak.
Groups of parents and children later gathered outside the burned-out garage to pay their respects. The families left tributes and said quiet prayers.
Floral tributes with messages written by children were left by the charred garage door. One group of children said: "He was a great friend and he loved to play football, any sport and computers."Reuse content