Is the marriage valid if the man committed fornication with a Christian woman, then they married, then the woman embraced Islam after the marriage, and they both repented from fornication, and they have two children?

The questioner is congratulated on his wife embracing Islam. It is to be known that Islamic law permits marriage to a chaste woman from among the People of the Book. However, if she is not chaste, then marriage to her is not permissible.

Based on this, the aforementioned marriage is invalid, and the marriage contract must be renewed. The two children are attributed to you if you believed this marriage to be valid, for Muslims are in agreement that any marriage which the husband believed to be permissible, if he engaged in intercourse within it, his child born thereof is attributed to him.

One must beware of zina (unlawful sexual intercourse) and its causes. A Muslim should ask the people of knowledge about what he is ignorant of before undertaking an action.