John Gill Commentary Numbers 31:18

"But all the women-children, that have not known man by lying with him, keep alive for yourselves." — Numbers 31:18 (ASV)

But all the women children
The females among the little ones:

that have not known a man by lying with him ;
which might be pretty clear, and easily concluded, from their age:

keep for yourselves ;
either to be handmaids to them, or to be married among them when grown up, and become proselytes, and initiated into their religion.