The Qur'an and Marrying Little Girls

Islam does allow you to Marry pre-menstruating girls. The following verse is from At-Talaq (or Divorce). Islam's main concern during a divorce is knowing who the father is (in case of a pregnancy). The waiting period is known as iddah.

65.4 Such of your women as have passed the age of monthly courses, for them the prescribed period, if you have any doubts, is three months, AND FOR THOSE WHO HAVE NO COURSES (it is the same): for those who are pregnant, their period is until they deliver their burdens: and for those who fear Allah, He will make things easy for them.

Tafsir al-Jalalayn (Commentary)
And [as for] those of your women who (read allā’ī or allā’i in both instances) no longer expect to menstruate, if you have any doubts, about their waiting period, their prescribed [waiting] period shall be three months, and [also for] those who have NOT  YET MENSTRUATED, because of their YOUNG AGE, their period shall [also] be three months — both cases apply to other than those whose spouses have died; for these [latter] their period is prescribed in the verse: they shall wait by themselves for four months and ten [days] [Q. 2:234]. And those who are pregnant, their term, the conclusion of their prescribed [waiting] period if divorced or if their spouses be dead, shall be when they deliver. And whoever fears God, He will make matters ease for him, in this world and in the Hereafter.

Tafsir Asbab Al-Nuzul by Al-Wahid
(And for such of your women as despair of menstruation…) [65:4]. Said Muqatil: “When the verse (Women who are divorced shall wait, keeping themselves apart…), Kallad ibn al-Nu‘man ibn Qays al-Ansari said: ‘O Messenger of Allah, what is the waiting period of the woman who does not menstruate and the woman who has not menstruated yet? And what is the waiting period of the pregnant woman?’ And so Allah, exalted is He, revealed this verse”. Abu Ishaq al-Muqri’ informed us Muhammad ibn ‘Abd Allah ibn Hamdun> Makki ibn ‘Abdan Abu’l-Azhar Asbat ibn Muhammad Mutarrif Abu ‘Uthman ‘Amr ibn Salim who said: “When the waiting period for divorced and widowed women was mentioned in Surah al-Baqarah, Ubayy ibn Ka‘b said: ‘O Messenger of Allah, some women of Medina are saying: there are other women who have not been mentioned!’ He asked him: ‘And who are they?’ He said: ‘Those WHO ARE TOO YOUNG [such that they have not started menstruating yet], those who are too old [whose menstruation has stopped] and those who are pregnant’. And so this verse (And for such of your women as despair of menstruation…) was revealed”.

Islamic Websitehttp://www.islamqa.com/en/ref/12667
“And those of your women as have passed the age of monthly courses, for them the ‘Iddah (prescribed period), if you have doubt (about their periods), is three months; and for those who have no courses [(i.e. they are still immature) their ‘Iddah (prescribed period) is three months likewise…”

Tafsir ibn Kathir (Read at your own leisure)
http://www.tafsir.com/default.asp?sid=65&tid=54223