Actual Events
Compared with the film's version of the story, the actual events of the "Affair of the Four Abandoned Children of Nishi-Sugamo" upon which the movie is based were far more grisly.
Originally, the mother had five children, two boys and three girls. One of the boys died from illness, but because none of the five children officially existed (she had not registered the births of any of them, thus none of them went to school), she wrapped the body up in some plastic sheets with some deodorizer and hid it in a closet.
When she left her children to live with her new lover, the oldest boy was 14, and he had three younger sisters, aged 7, 3 and 2.
The older boy took to bringing over two friends ("A" and "B" in the report) he'd made, but these kids were a lot nastier than the ones depicted in the film. Ultimately, one of them (B) became angry at the two-year-old girl for eating a bowl of ramen he'd brought over, and beat her up, ultimately killing her.
The oldest son and friend (A) packed the body up and took it to some mountains nearby, and buried it in a shallow grave. This was in April 1988.
In July of that year the landlord finally realized that the apartment seemed to be occupied only by children, and called the police, who found the two remaining girls badly undernourished. Searching the apartment, they discovered the corpse that the mother had hidden.
The story was covered very heavily by the mass media; and the mother, seeing this on the news, wondered if it was all about her children, and turned herself in to the police.
The eldest boy was not in the room when his sister died, but he knew what his friend was capable of, and had criminal charges against him which were later dropped. The two younger surviving children were eventually returned to their mother's care. The children's true names were never released to the media.