Is it permissible for a father to give charity from his own money to purify his children's money of the usurious interest deposited in the savings account, or must the charity be given from the usurious money itself?

It is permissible for the father to dispose of the usurious interest by spending an equivalent amount from his other funds on charitable causes and public welfare. It is not a condition to dispose of the usurious interest itself. In this case, it becomes permissible to benefit from all the money that the father deposited in savings accounts, because money is not designated by specific allocation, and the prohibition of usurious interest relates to the obligation (dhammah), not to the specific physical money itself.