Those in charge knew that the long term threats were not real. Otherwise the army would not have allowed our whole family to move to Japan 4 years after the bomb was dropped.
My father was stationed just 40 miles away from Hiroshima and our whole family lived there from 1949 to 1951. We visited Hiroshima and even met the mayor in the famous building shown so often representing what was left of the city. It was ground zero. Hundreds begged just outside that building and showed their burned skin and permanent wounds obtained during the war. I’m now 76 and cancer free. Brother is 80 no health issues. Mom lived to 95, Dad to 84 of natural causes.
Sensitivity or resilience to exposure to radiation is not evenly distributed among the population - rather the ability to resist the damaging effects of radiation (repair the body) follows a bell shaped curve distribution - sounds like your family was on the “winning” side of that distribution.
Ironically, the man personally responsible for the Chernobyl disaster was the only survivor among the group of scientists directly exposed to radiation in the control room - the others died grisly deaths but he recovered to live to a ripe old age.
Sadly, the US government would allow people into an irradiated zone. They sent the crew of a navy ship to assist the Japanese with Fukushima immediately following the event -they sailed into the plume and had crewmen “decontaminate” the deck without wearing protective gear. Many sailors have since fallen ill. The former prime minister of Japan said the following about the navy crewmen sickened by their exposure while trying to assist the Japanese:
“Speaking at a news conference Tuesday in Carlsbad, California, with some of the plaintiffs, Koizumi said, “Those who gave their all to assist Japan are now suffering from serious illness. I can’t overlook them.””