Let us look at the Grand Ayatollah Khomeini's ruling on the matter:
A man can marry a girl younger than nine years of age, even if the girl is still a baby being breastfed. A man, however is prohibited from having intercourse with a girl younger than nine, other sexual acts such as foreplay, rubbing, kissing and sodomy are allowed. A man having intercourse with a girl younger than nine years of age has not committed a crime, but only an infraction, if the girl is not permanently damaged.
The use of the word "marry" in the above text is also interesting. In the Christian context "marry" means for life (until death does you part) but Islam has mutah, a temporary marriage, sometimes as short as a couple of hours. Usually a "dowry" is payable to a girl's parents when she is mutah'ed for the purpose of mufa'kathat.