Are the children of my ex-husband considered Mahrams (unmarriageable relatives) for me?
When a man marries a woman, his sons, grand-sons, and those lowest in line of descent become Mahrams for her. Whether they are born before she marries their father or after that, or even after being divorced, she remains permanently forbidden in marriage to any of them. Allah (Glorified and Exalted be He) states,
And marry not women whom your fathers married, except what has already passed; indeed it was shameful and most hateful, and an evil way.
(Part No. 17; Page No. 348) May Allah grant us success and peace and blessings be upon our Prophet, his family and Companions.