I just read Numbers Chapter 31.
Numbers 31:18 does not say anything about child-rape.
It would have been completely 100% against the Law of Moses.
It is way, way more likely that the girls were taken as servants or slaves -- nothing is mentioned of child-rape in this Verse.
Numbers 31:18:
(King James Version) But all the women children, that have not known a man by lying with him, keep alive for yourselves.
(American Standard Version) But all the women-children, that have not known man by lying with him, keep alive for yourselves.
(Revised Standard Version) But all the young girls who have not known man by lying with him, keep alive for yourselves.
(World English Bible) But all the girls, who have not known man by lying with him, keep alive for yourselves.
The fact that only the virgin girls were saved, is, I believe, because that would signify that they were not fornicators.
They may have just saved all girls under a certain age.