Anko3626: A Summer Night's event
One hot night, a Koreimu makes a lot of noise, screaming it can't sleep because of the heat. The mother Reimu tells it to be quiet, and offers to rub-rub, but Reimu insults it, saying that doing rub-rub would make it even hotter, and then it can't take it easy.
Outside, a Marisa and a Komarisa are going through the same thing, and the parent is busy spanking the child. The whiny, demanding koyukkuri finally crosses the line when it says that it wanted to be born from a parent that isn't so useless, not knowing how much trouble Marisa goes through to bring back food for it. Marisa hits the ko hard, and it screams in pain.
Yet it still continues to insult Marisa, until it decides to kill the koyukkuri. Surprisingly, the Komarisa counter that yukkuri who kill other yukkuris can't take it easy, and will be punished, and using that, it insults the parent further. But Marisa finds a loophole, it wouldn't kill the koyukkuri, but it would punish it by not letting it take it easy.
With that, Marisa grabs the child's hat and starts ripping it to pieces, then it does the same to the child's braid. Eventually the child realizes it can't do anything about it, and apologizes tearfully, saying how it's scum who's useless and can't hunt, the same insults it hurled at its parent earlier.
Inside, Reimu has done the same to the Koreimu, who had screamed about its (insert complimentary descriptions here) ribbon and braid, leaving the koyukkuri bald and crying, and also apologizing.
That's when it's revealed that they're all part of the same family, when Marisa goes back into the box that is their home. When Marisa and Reimu see the koyukkuri that was with the other, they immediately understand what happened, and smile at each other, recognizing the fact that they have to be harsh-hearted when raising children as the bald children tremble in fear.
That event wasn't only restricted to this family. The heat made a lot of whiny koyukkuris learn why they should just shut up and bear the heat; there isn't one who isn't unharmed in any way. Some parents killed their kos, but escaped in fear of reprisals. But they can always make more if the kos die.
It is something that will happen eventually, an event that destroys the relationship built between the parents and the children. The reason this time is the heat, but it may be due to a different reason next time.