In my answer to the linked Question, I did not elaborate on conditions where Slavery is permitted. I will try not to go off-topic, so let me elaborate.
The broader definition of crime includes breaking of treaties, this is "inter-national" crime conducted by either signatories. This therefore permits Muslims to declare war and thereafter having the option of capturing the criminals as slaves.
With the above being said, the same applies to criminals such as the cartel, who have created their own armies. The gangs that infest America, who have created their own armies also, warring against the state, and against the people.
Prisons are filled with such cases. US prison population for example is, almost the same size as NZ population. The expenses of these prisoners are paid by the victims (tax payers). Imagine your daughter is killed, and then your tax money goes to feed, cloth, house and entertain the monster that killed her. I'm sorry that you find that rational, or justified, or logical.
Consider the fact that slaves in Muslim societies did not meet the same horrid situation as slaves in some other societies.
"I heard Sulaim bin 'Amir saying: 'There was a treaty between
Mu'awiyah and the people of Rome. He was making an expedition into
their lands so that when the period of the treaty was expires he would
attack them. So when a man upon an animal' - or - 'upon a horse said:
"Allahu Akbar! Fulfillment not betrayal!" - and it turned out to be
'Amr bin 'Abasah - Mu'awiyah asked him about that. He said: "I heard
the Messenger of Allah (ﷺ) said: 'Whoever has a treaty between himself
and a people, then let him not violate the treaty nor try to change it
until its time has passed, or , in retribution for a similar
offense.'" He said: "So Mu'awiyah returned with the people."
You can't go to war unless treaty is violated, or there is no treaty between the two parties. These are conditions which allows declaration of war. Without declaration of war, no one can be taken as slave, neither women, nor children, nor men. So yes, breaking a treaty is a requirement.
Islam doesn't deal with a nation at individual human level, rather as a whole. Just like US didn't deal with the Japanese at individual human level when Japan was nuked. There are special cases though.
Regarding children "not breaking" the treaty. Those children killed in the nuking of Japan didn't break any treaty either. The comparison is made to help you better understand.