She was not a child at the time of her marriage despite many assumptions.
In the booklet Prophet of Islam, which was later incorporated in 1948 as the first chapter of his book Living Thoughts of the Prophet Muhammad, Maulana Muhammad Ali writes in a lengthy footnote as follows:
“A great misconception prevails as to the age at which Aisha was taken in marriage by the Prophet. Ibn Sa‘d has stated in the Tabaqat that when Abu Bakr [father of Aisha] was approached on behalf of the Holy Prophet, he replied that the girl had already been betrothed to Jubair, and that he would have to settle the matter first with him. This shows that Aisha must have been approaching majority at the time. Again, the Isaba, speaking of the Prophet’s daughter Fatima, says that she was born five years before the Call and was about five years older than Aisha. This shows that Aisha must have been about ten years at the time of her betrothal to the Prophet, and not six years as she is generally supposed to be. This is further borne out by the fact that Aisha herself is reported to have stated that when the chapter [of the Holy Quran] entitled The Moon, the fifty-fourth chapter, was revealed, she was a girl playing about and remembered certain verses then revealed. Now the fifty-fourth chapter was undoubtedly revealed before the sixth year of the Call. All these considerations point to but one conclusion, viz., that Aisha could not have been less than ten years of age at the time of her nikah, which was virtually only a betrothal. And there is one report in the Tabaqat that Aisha was nine years of age at the time of nikah. Again it is a fact admitted on all hands that the nikah of Aisha took place in the tenth year of the Call in the month of Shawwal, while there is also preponderance of evidence as to the consummation of her marriage taking place in the second year of Hijra in the same month, which shows that full five years had elapsed between the nikah and the consummation. Hence there is not the least doubt that Aisha was at least nine or ten years of age at the time of betrothal, and fourteen or fifteen years at the time of marriage.”
To facilitate understanding dates of these events, please note that it was in the tenth year of the Call, i.e. the tenth year after the Holy Prophet Muhammad received his calling from God to his mission of prophethood, that his wife Khadija passed away, and the approach was made to Abu Bakr for the hand of his daughter Aisha. The hijra or emigration of the Holy Prophet to Madina took place three years later, and Aisha came to the household of the Holy Prophet in the second year after hijra. So if Aisha was born in the year of the Call, she would be ten years old at the time of the nikah and fifteen years old at the time of the consummation of the marriage.
For details, please see the following link, by Dr. Zakir Naik