For those who don't know the story of Banu Qurayza, I can point to the wikipedia article which, while not being my primary source, is sufficiently well documented to serve as a first source about this subject to someone who wants to get acquainted with it.
To summarize for those who don't know, in the siege of the Trench, the Muslims came closer to complete defeat and anihilation than ever before because, in part, of the treacherie of one of the Jewish tribes of Medine called Banu Qurayza. The Muslims ultimately won when the Meccians retreated (due to a storm) and Banu Qurayza were punished by killing all the adult males, dividing their posessions and enslaving their women and children. This is usually explained by the immense danger that the Muslims faced and the need for an exemplary retribution.
Some accounts say that the "men" were distinguished from the "boys" by checking pubic hairs. Some accounts say 600 men were slain, some talk about as high a number as 900. I am willing to concede that it was only 2 men and that they were all 30 years old. I have a habit of avoiding debates on shaky basis: If there is more than one source or account, I simply do not debate it.
As such, I am not concerned with the number of men that were killed, or their age. As the title suggests, this is a question about the enslavement of the rest.
I am also willing to say that all the women were grown women and, without exception, may have deserved to be enslaved (I am, of course, not of such a mind, but I am trying to limit the scope of the question so as to avoid useless or off-topic answers).
But the children? In light of this question, it seems clear that, in the afterlife, children will not be punished for the sins of their parents, but what of this life?
Let us be clear, while there are multiple contradictory sources on the number of deaths, or the identity of the executioners, there is not a single source that does not agree with the general and unavoidable consensus that the children were enslaved. I have been studying religion for well over 15 years now, and was troubled by this story (and others similar to this one) for a long time.
There are some historians that say that it was common practice in the Arab peninsula, and some others who say that it was not. Some other historians say that it was Jewish law, but taking the loser's women and children was also carried out for non-Jewish by Muslim armies on other occasions. But regardless of whether it was custom, or Jewish law, the fact that it was carried out and accepted by the prophet is a validation of that practice, making it Islamic.
My questions are these:
How can the punishing of the children for the sins of their parents be considered just.
Is there any account about these children ? Meaning, when reaching an adult age, were they offered freedom ?
Is there any later hadiths forbidding enslaving children in the context of war ?
A side note: Please avoid comparing "Islamic slavery" to "real slavery" (it was better, the slaves had rights, they were treated well, there are a lot of ways they can be freed, it was a lot worse in the Americas, etc., I have had these debates before, this is not the place for it, and it was vastly addressed in this question and in this one). Whether or not the enslaved children were fed honey and grapes and slept on feathers is of no relevance to the question.