Is the money inherited by the father from his grandfather, which was deposited in an interest-bearing bank and whose Zakat was not properly paid annually, considered forbidden money, and are the children sinful for the father spending on them from it, and how can this money be purified, and is it permissible to pay Zakat from it without the father's knowledge?

It is permissible to benefit from money that is mixed with lawful and unlawful elements, to the extent of the lawful portion within it. If this money is inherited, the unlawful part must be disposed of, and zakat must be paid from the principal (not from the usurious profits) before distribution.

It is not permissible to take the amount of zakat from the father's money during his lifetime through deception and without his knowledge.

The father must be advised and warned of Allah's punishment if he persists in dealing with usury and withholding zakat.