Even today's (self-described) enlightened societies draw a distinction between the lawful killing of the children of enemy nations (as "unavoidable consequences" of war) and the unlawful killing of children by their own parents (at least once those children are outside the womb).

Unfortunately, at the time of the Canaanite cities, there wasn't really much infrastructure for providing welfare services to war orphans, and there was a very real risk that young males of conquered peoples would grow up to seek genocidal revenge on their conquerors. By that standard, offering homes to the female prisoners of war was relatively compassionate.