The two young brothers looked into the bright blue sky and waved happily at the shiny plane flying far above.
Their sister, Emiko Okada, eight years old at the time, remembers an intense flash of light and her mother suddenly rushing into the yard to the children, bleeding from where shards of glass had lodged in her head.
Next came the fire - and people running, hair standing on end, white bones exposed, skin and flesh burning. People were vomiting, not just blood but black ooze from their nose and mouth.
This was the day the A-bomb fell on Hiroshima.
Ms Okada is one of a dwindling group of survivors from that morning in 1945, determined never to let the terrible human cost of nuclear war be forgotten, even after they are gone.
''Frightening is not the world I can use, it was something much worse,'' she said via a translator, still upset by the memory.
Stories from the survivors, known in Japan as hibakusha, were told to foreign ministers of a 12-nation group, including Australia, gathered in Hiroshima at the weekend to kickstart global talks on nuclear disarmament.
Survivors' stories are being preserved in an online archive by the Tokyo Metropolitan University.
Ms Okada lived on the red edge of the blast zone, where an estimated 70,000 people died instantly when the bomb exploded over the city. Her older sister had gone downtown in Hiroshima that morning and was never seen again.
But that was just the beginning. Up to another 70,000 are thought to have perished from radiation sickness in the months afterwards.
''Those of us that survived went through a different kind of suffering,'' Ms Okada said.
Her ailments were common to hibakusha - hair loss, fevers, and a dull feeling of constant fatigue. Many suffered blood disorders and cancer. Her two brothers, aged five and three at the time, had burns to their upraised arms from heat rays estimated to have reached 4000 degrees at the source. Her mother died 14 years later with glass fragments still in her skull.
The terrible consequences of radiation poisoning also struck rescuers and passed to a new generation of unborn children in their mother's womb.
Kosei Mito was born in January 1946, five months after the bombing. His mother had returned to the radiation zone a few days after the attack to find the family's house destroyed with much of the city.
Mr Mito suffered as a result. ''I was very sickly,'' he said, and was twice struck down with scarlet fever. But he now shares his story as a guide around the Hiroshima peace park, eager to warn tourists about the danger from nuclear annihilation.
Ms Okada has the same goal, and has travelled from Poland to Pakistan and countries in between to tell school students about the devastating cost of atomic war.
The global nuclear arsenal, despite reductions since the end of the Cold War, is about 17,000 bombs.
''Only one was needed to bring such pain and suffering to the people of Hiroshima,'' Ms Okada said.
Daniel Flitton travelled to Japan courtesy of the Japanese government.
The story Hiroshima survivors offer peace and hope for nuclear disarmament first appeared on The Sydney Morning Herald.