What is the ruling on marrying an unmarried Christian mother, and is it permissible to marry her after she converts to Islam? What will be the fate of her son if she converts to Islam?

It is permissible to marry an unmarried Christian woman who has a child, provided she is chaste or has repented from fornication, though marrying a Muslim woman is preferable. If this woman converts to Islam, it is permissible to marry her if the conditions are met.

As for the fate of her son: if he was born from a valid marriage, he is attributed to his father. If he was born from fornication, he is attributed to his mother, and she is obligated to be good to him and invite him to Islam. If the mother converts to Islam and the child is young, he follows her in Islam.