Is uttering the word "kufr" (disbelief) in a state of anger by a young, unintentional person considered an act of disbelief, and is repentance obligatory upon him in this case?

If the questioner, by saying "even if he did not intend it," means a word of disbelief (kufr), then this is correct. For intending disbelief is not a condition for ruling according to the implication of a word of disbelief; rather, merely intending to utter the word itself suffices. However, if he did not intend the speech at all and it came out erroneously due to an impediment such as anger or absent-mindedness, then he is not held accountable for it.

As for the apostasy of a child: If he is not discerning (mumayyiz), then apostasy does not occur from him. If he is discerning and close to puberty (i.e., approaching الاحتلام), then there is a difference of opinion among scholars regarding its occurrence from him. In any case, repentance is accepted from the one who utters this word, whether he is an adult or an adolescent.