John Wesley on Numbers 31:17

The little ones - Which they were forbidden to do to other people, Deu 20:14, except the Canaanites, to whom this people had equaled themselves by their horrid crimes, and therefore it is not strange, nor unjust, that God, the supreme Lord of all mens lives, who as he gives them, so may take them away when he pleaseth, did equal them in the punishment. Every woman - Partly for punishment, because the guilt was general, and though some of them only did prostitute themselves to the Israelites, yet the rest made themselves accessary by their consent or approbation; and partly, for prevention of the like mischief from such an adulterous generation.