The handles had been removed from the door which could only be opened from the outside by a pair of pliers, the hearing at Preston, Lancashire, was told.
Daniel Bamford, four, and his brother, Michael, two, were found in bed in each other's arms in the room at their Preston home in September. A cigarette lighter was also found in the room.
Attempts to revive them failed and they were dead on arrival at hospital. Their mother Maureen, 21, told the inquest that her sons were 'mischievous' like other children of their age and kept coming in and out of their bedroom. Her sister had taken the handles off the door, but 'you could get in using a pair of pliers', she said.
Mrs Bamford agreed with Howard McCann, the coroner, that the boys were 'effectively locked in'. Dr Edmund Tapp, a Home Office pathologist, said they died from smoke inhalation. The court also heard that a file was being sent to the Crown Prosecution Service.
Recording verdicts of accidental death, the coroner said the likely cause of the fire was one of the children setting fire to some toys. But a key factor was the bedroom door being locked.