I simply find the taking of innocent lives to be abhorrent. Evidently you do not.
If a child grows up amongst an evil pagan people, known for relentless war-like aggressions, massacres, enslavements, human sacrifices, and all manner of unthinkably cruel punishments, that child will be trained to view such practices as normal and to be expected, and desirous to take part in a continuation of the very same hideous things. And God will punish such people in their eternity to come, as He knows precisely that they will never change. So, whether or not such people die by the sword, or whether they live into old age, upon death, their fate is the very same - due to God's righteous judgment. But Scripture gives us good reason to believe that children killed are instantly in the arms of the Lord. Why? Because what God desires of them, righteous behavior, turning to Him, they are not mature enough to know the importance of. They are not mature enough to understand their sinful ways, or the terrible sins of their society. So, for such a child, one moment immersed in such a culture, and the very next, a three second sword strike brings them into the glorious arms of the Lord - FOREVER! Is that not far more loving than to allow them to continue in their heathen culture of constant despicable acts - perhaps themselves even risking being sacrificed to pagan Gods? Left to their own culture and society, such children will undoubtedly adopt the very same beliefs and practices that would eventually spiritually doom them.
OK, some might say, of such pagan children - why KILL them? Why not adopt them into Israel's society? I don't have a good answer for that, other than to imagine that God wanted ALL vestiges of pagan beliefs snuffed out - and older children particularly would have had their thinking very much infected by such pagan beliefs and practices. But, again, the sword strike we see as so unthinkable, is but an instant, a mere second, and next would be the beginning of a pagan child's eternal bliss and joy. And ONLY God has the right to command ANY life be taken, because He is the Life Giver. And God perfectly knows every mature pagan's heart, as to whether or not they will ever repent of their ways and follow Him. So, God views their physical destruction as only momentary, as pagan adults killed will not enter into eternal fellowship with God. Of course, this is true of ALL whom fail to embrace the Lord. But like the Trinity, where many verses reveal it to be what Scripture teaches, this idea of children being saved at death is implied by various places in Scripture. So, whether a pagan (whom God knows will never repent, never embrace Him) lives into a life of old age and dies in his sleep, or whether he is killed in an attack ordered by the Lord, God is focused on what He sees as being truly important - their eternal fate. And to God, HOW a pagan dies is of little importance, because their eternal fate is the real tragedy - and one brought on by their own choices as to belief and practices.