Would a girl accept marrying a man who is already married and has a son, despite her family's and society's reservations about the idea of a second wife, knowing that she feels comfortable with him and sees him as suitable for raising her children?

There is no objection for a girl to marry a married man (as a second, third, or fourth wife), as this is permissible in Islamic law and there is nothing blameworthy about the girl in doing so. The best of women have done this. If the suitor is a man of good religion and character, accept him after seeking guidance from Allah, and do not pay attention to people's opinions. Fear Allah in helping your husband be just among his wives.