Christians believe that the Bible has nothing to say with regards to the condoning or approving of child marriage. However, when we look into the Bible, we find passages that do approve of it. One of the most prominent examples is of Numbers 31.
17 Now therefore kill every male among the little ones, and kill every woman that hath known man by lying with him.
18 But all the women children, that have not known a man by lying with him, keep alive for yourselves. Numbers 31:17-18 (KJV)
This is one of the most disturbing set of passages that I have come across in the Bible. These passages basically state to Moses’s soldiers that it’s ‘OK’ to have sexual relations with “women children” that are obtained in war. The translation for “women children” is very misleading and it has been deliberately translated that way to deceive readers. In fact, when we look at the Hebrew words used for the verse, it just says female “children”, “little ones”. For further details that the passage speaks about children and not women click on the following article here.
Three accurate translations which clearly show that the passage only speaks about ‘female children’ are as follows:
Jubilee Bible 2000 – “But all the female children that have not known a man by lying with him keep alive for yourselves.” – Numbers 31:18
Webster’s Bible – “Translation But all the female children that have not known a man by lying with him, keep alive for yourselves.” – Numbers 31:18
Living Bible (TLB) – “Only the little girls may live; you may keep them for yourselves.” – Numbers 31:18
Since it’s clear that Numbers 31:18 speaks about female children and not ‘women’, what does “keep alive for yourselves” mean? Of course the verse gave permission to Moses’s soldiers to marry those female children. However, don’t take my word for it, let’s see what Christian Bible commentaries say for the verse.
Whedon’s Commentary on the Bible:
The little ones — The object of the command to kill every male was to exterminate the whole nation, the cup of whose iniquity was full. For the righteousness of the mode see Joshua 6:21, note. Every woman who might possibly have been engaged in the licentious worship of Peor was to share the fate of the male children, to preserve Israel from all taint of that abomination.
The pure maidens could be incorporated into Israel without peril to the national religion. Joshua 6:23-25, notes. They could not be treated as concubines, since the law against fornication was in full force, (Deuteronomy 22:25-29,) but they could be lawfully married to their captors (Deuteronomy 21:10-14). 
Peter Pett’s Commentary on the Bible:
‘But all the women-children, who have not known man by lying with him, keep alive for yourselves.’ But the young women children were a different matter. They could be taken in marriage or as servants and would expect to accept the religion of their new husbands or masters. They would gradually be merged into Israel.
Compare Deuteronomy 21:10-14. They would not feel the same responsibility for blood vengeance which was mainly incumbent on the males.
NIV: The New American Commentary – An Exegetical And Theological Exposition of Holy Scripture:
… Only the young girls would be allowed to live so that they may be taken as wives or slaves by Israelite men, according to the principles of holy war (Deut 20:13-14; 21:10-14). By this they could be brought under the umbrella of the covenant community of faith. 
John Dummelow’s Commentary on the Bible:
13-18. The male children are put to death in order that the race of idolaters may be extirpated. The older women are also slain as having been the prime cause of the apostasy, and likely to lead the people astray at a future time.
The women-children, or young females, are spared, and are taken as slaves or wives, being probably adopted into the Hebrew nation as proselytes: cp. Deuteronomy 21:10-14. For the reason of such wholesale slaughter see Numbers 33:55; Deuteronomy 20:17-18; Joshua 23:13 and on Numbers 25:16-18. 
The above Bible commentaries quite clearly state that, “keep alive for yourselves” means those female children were to be married off to grown adult Israelite men. There is no mention anywhere in the commentaries that the girls have to grow up and hit puberty before they get married. I brought up the latter point, because some desperate missionaries claim that the girls had to grow up before a marriage took place. Numbers 31:18 in the Bible shows that Moses’s soldiers were commanded to marry pre-pubescent girls. This means that the holiest text of Christianity supports marrying and having sexual relations with pre-pubescent girls. This is concrete proof that the Bible endorses marriage with pre-pubescent girls. As we have read Numbers 31:18 and the Bible commentaries, they all state that marriage took place straight away.
In fact, in the Biblical commentaries already mentioned, they all refer to Deuteronomy 21:10-14 as evidence that the marriage took place straight away within a month. Let’s read the verse:
Deuteronomy 21:10-14 Good News Translation (GNT) – “When the Lord your God gives you victory in battle and you take prisoners, 11 you may see among them a beautiful woman that you like and want to marry. 12 Take her to your home, where she will shave her head, cut her fingernails, 13 and change her clothes. She is to stay in your home and mourn for her parents for a month; after that, you may marry her. 14 Later, if you no longer want her, you are to let her go free. Since you forced her to have intercourse with you, you cannot treat her as a slave and sell her.”
In the part where it says beautiful “woman”, the Hebrew word ‘ishshah’ is translated as ‘woman’ and it is also translated as “female” (when we consult Jubilee Bible and Webster’s Bible for Numbers 31:18, both of them translate the Hebrew word ‘Ishashah’ as ‘female’).
Reading Deuteronomy 21:10-14 we get a clear picture that YHWH commands its soldiers that when they go out in war and they see a beautiful females, and the soldier is attracted to her, he can “TAKE” the female and force her to marry him after one month. The word “take” is in of itself evidence that the females were taken without their consent i.e., they were forced into a marriage that they did not consent to. In verse 13 to 14 it goes further and states, that if the soldier were to not desire the female any-more he may let her go as he wishes because “you forced her to have intercourse with you”.
The period in which the soldier cannot marry her is within a one month period of mourning. After that period the soldier can do what he wishes to the female, as the verse clearly states.
 Whedon’s Commentary on the Bible
 Peter Pett’s Commentary on the Bible
 NIV: The New American Commentary – An Exegetical And Theological Exposition of Holy Scripture: Numbers, [Copyright 2000 – B& H Publishing Group] By R. Dennis Cole, volume 3b, page 499
 John Dummelow’s Commentary on the Bible