What should a man do who married a woman and had children with her, then discovered she is his father's wife, and what is the ruling concerning their children?

A man must divorce his wife immediately upon learning that she was his father's wife, because she is permanently forbidden to him by Allah's saying: "And do not marry those [women] whom your fathers married." As for the children, they are legitimate and are attributed to him, because the marriage contract was valid at the time it was concluded, and its invalidation began from the moment he learned the truth of the matter.