Commentary for Numbers 31:18

החיו לכם, "keep alive for yourselves." The meaning is that those who were allowed to live should convert to Judaism and only when they were Jewish would they qualify for the description חיים, "to be alive," as then they would be allowed to get married and to remain "alive" by means of the children they would bear.

'וכל הטף בנשים אשר לא ידעו איש וגו, “but any children among the women who have not had carnal knowledge, etc.” you shall let live for your benefit. This formulation prompted Rabbi Shimon ben Yochai to state that a female convert who has not attained the age of three years and a day, is fit to marry a priest. [as she could not have been contaminated through carnal intercourse] (Sifri on this verse)