Must the children dispose of the usurious interest and pay zakat on their father's money that was deposited in a usurious bank and spent on building a house and a gift to the children, and are the children obligated to pay that from their own money?

Usurious gains must be disposed of and spent on the welfare of Muslims. As for zakat, it is due only on the principal amount for the past years before the purchase of the building. It is obligatory upon the father, as he was the owner of the money during that period. Money, after being spent on a building intended for residence, is not subject to zakat unless it is rented out. In that case, the rent is subject to zakat once it reaches the nisab and a hawl has passed.