What is required of a girl who vowed not to have children, then desired to have them: must she adhere to her vow, or can she expiate it and have children? And what is the expiation if it is obligatory?

A vow is not binding nor does anything result from it, except for that which is an act of devotion, based on the saying of the Prophet, peace and blessings be upon him: "There is no vow except in that by which the Face of Allah is sought." It is recommended to offer an expiation for an oath (kaffarat yamin), which is feeding ten poor persons, or clothing them, or emancipating a slave. If none of these are available, then fasting three days.