After the Jews were defeated at Khaybar, the Prophet (SAWS) approached Safiyyah bint Huyyay and gave her two options: convert to Islam and marry him or remain a Jew and go back to her people. She chose to convert to Islam and marry him:
Islam Today: Did Prophet Muhammad force Safiyya bint Huyayy into marriage after killing her father?
www.alhakam.org
So it is clear she converted to Islam and married the Prophet (SAWS) due to genuine beliefs and convictions, not because she was forced since the Prophet (SAWS) gave her a clear choice.
As for the child marriage argument, this has been addressed so many times. I think this guy sums it up well, so I encourage you to watch his video: