There's extensive reputable hadiths about this, confirming she was still child. One records a complaint that a child was in the Muslims' battlecamp (religion of peace etc) in breach of rules, to which it was explained it was the "Prophet's" wife so ok.
Furthermore, the Quran itself confirms that "rape", "murder" and "theft" are ok if done by Muslims, not to mention multiple wives, so it would hardly be out of character for the great man himself to add one more so-called "crime" to the list.