Taliban insist the slaughter on children was absolutely according to the Quran

Killing of women and children is also in accordance with the teachings of the Prophet. Those who object to this claim can refer to Sahih Bukhari, Volume 5, Hadith 148.

– Taliban Spokesperson Muhammad Khorasani

Sahih Bukhari, Volume 5, Book 58, Number 148:

Some people (i.e. the Jews of Bani bin Quraiza) agreed to accept the verdict of Sad bin Muadh so the Prophet sent for him (i.e. Sad bin Muadh). He came riding a donkey, and when he approached the Mosque, the Prophet said, “Get up for the best amongst you.” or said, “Get up for your chief.” Then the Prophet said, “O Sad! These people have agreed to accept your verdict.” Sad said, “I judge that their warriors should be killed and their children and women should be taken as captives.” The Prophet said, “You have given a judgment similar to Allah’s Judgment (or the King’s judgment).”

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They seem to be assuming the kids were old enough to be classified as adults. Otherwise, it was a textbook case of tribal rules: kill the men, enslave the women and children.  You can read similar things in Homer’s Iliad (estimated at eight century BC).   

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