They were in the kitchen when their house collapsed but the teenager was able to reach food from the refrigerator, helping them survive for nine days, broadcaster NHK quoted rescuers as saying.
But with half a million tsunami survivors huddled in threadbare, chilly shelters and the threat of disaster at the Fukushima No. 1 nuclear plant stretching frayed nerves, the mood in the world's third-biggest economy remained grim.
Food contaminated with radiation was found for the first time outside Japan -- where milk and spinach have already been tainted by a plume from Fukushima -- as Taiwan detected radioactivity in a batch of imported Japanese fava beans.
The discovery of traces of radioactive iodine in Tokyo tap water, well to the southwest of the crippled atomic power plant on the Pacific coast, compounded public anxiety but authorities said there was no danger to health.
The Fukushima plant was struck on March 11 by the 9.0-magnitude earthquake and tsunami which, with 8,450 people confirmed killed, is Japan's deadliest natural disaster since the Great Kanto quake levelled much of Tokyo in 1923.
Another 12,931 are missing, feared swept out to sea by the 10-metre (33-foot) tsunami or buried in the wreckage of buildings.
In Miyagi prefecture on the northeast coast, where the tsunami reduced entire towns to splintered matchwood, the official death toll stood at 5,053, but the police chief warned that the number could eventually rise to 15,000.
Cooling systems that are meant to protect the Fukushima plant's six reactors from a potentially disastrous meltdown were knocked out by the massive waves, and engineers have since been battling to control rising temperatures.
Radiation-suited crews were striving to restore electricity to the ageing facility 250 kilometres (155 miles) northeast of Tokyo, after extending a high-voltage cable into the site from the national grid.
Engineers were checking the cooling and other systems at reactor No. 2 late Sunday, aiming to restore the power soon, operator Tokyo Electric Power Co (TEPCO) said.
An external electricity supply has been restored to the distributor but power at the reactor unit was not yet back, spokesman Naohiro Omura said.
"It will take more time. It's not clear when we can try to restore the systems," he said.
Fire engines earlier aimed their water jets at the reactors and fuel rod pools, where overheating is an equal concern, dumping thousands of tonnes of seawater from the Pacific.
"Our desperate efforts to prevent the situation worsening are making certain progress," said chief government spokesman Yukio Edano.
"But we must not underestimate this situation, and we are not being optimistic that things will suddenly improve," he told a news conference.
Defence Minister Toshimi Kitazawa said the temperature in all spent fuel-rod pools at the facility had dropped below 100 degrees Celsius (212 degrees Fahrenheit) -- suggesting water cooling operations were having some effect.
Authorities said reactors five and six at the Fukushima complex meanwhile were in "stable condition", Kyodo News reported.
Six workers at the plant have been exposed to high levels of radiation but are continuing to work and have suffered no health problems, TEPCO said.
The UN's atomic watchdog Sunday noted "some positive developments" at the plant over the past 24 hours, but warned that the crisis there remained serious.
Prime Minister Naoto Kan was supposed to visit a staging ground for the Fukushima relief efforts on Monday, as well as the city of Ishinomaki, where the two survivors were found.
But he had to cancel his trip because of rain which threatened to pile more misery on thousands of homeless survivors -- including many elderly and children -- who have had to battle through biting cold in makeshift shelters.
According to the charity Save the Children, around 100,000 children were displaced by the quake and tsunami, and signs of trauma are evident among young survivors as the nuclear crisis and countless aftershocks fuel their terror.
"We found children in desperate conditions, huddling around kerosene lamps and wrapped in blankets," Save the Children spokesman Ian Woolverton said after visiting a number of evacuation centres in Japan's northeast.
"They told me about their anxieties, especially their fears about radiation," Woolverton said, adding that several youngsters had mentioned the US atom bomb attacks on Hiroshima and Nagasaki, which they know from school.
The government has insisted that there is no widespread threat of radiation. But the discovery of the tainted fava beans by Taiwanese customs officers will do nothing to calm public anxiety that has already spread far beyond Japan.
Several governments in Asia have begun systematic radiation checks on made-in-Japan goods, as well as of passengers arriving on flights from the country.
But Tsai Shu-chen of Taiwan's Food and Drug Administration stressed that the radioactive iodine and caesium-137 found on the fava beans were well below legal safety levels.
In the disaster epicentre, authorities have been battling to get more fuel and food to survivors enduring freezing temperatures.
At shelters, some grandparents are telling children stories of how they overcame hardships in their own childhood during and after World War II, which left Japan in ruins.
"We have to live at whatever cost," said Shigenori Kikuta, 72.
"We have to tell our young people to remember this and pass on our story to future generations, for when they become parents themselves."