Is it permissible for children to benefit from their father's money, which he earned from his job as a financial manager in a company that deals with usury, knowing that he used to sign off on the company's usurious transactions and believes that only personal usury is forbidden, and that all their possessions came from this money?

It is not permissible to sign usurious transactions because it falls under cooperation in forbidden sin. There will be an unlawful portion in the salary earned from his work, proportionate to that work. However, as long as he was ignorant, there is no sin upon him, but he must cease from it. As for his children benefiting from his money and residing in his house, there is no harm in it because his money is not entirely unlawful. What corresponds to the percentage of the unlawful, if he was ignorant of it, is permissible for him. This is based on the Almighty's saying: "And Allah has permitted trade and forbidden usury. So whoever has received an admonition from his Lord and desists may have what has already passed, and his affair is with Allah. But whoever returns [to usury] – those are the companions of the Fire; they will abide eternally therein. Allah destroys usury and gives increase for charities. And Allah does not like every sinful disbeliever."