What is the ruling concerning money placed in the bank in the names of children, of which one part consists of the father’s earnings and another part of bank interest? Is zakat obligatory on the principal amount or on the interest only? And what should the questioner do with the remaining money that exceeds the nisab, but cannot be disposed of except with the father's permission?

If the money registered in your name in the bank from your father is a gift and has been legally acquired, then it is your property, and Zakat is obligatory on it if a year has passed and it has reached the Nisab. However, if it is merely a will to be executed after his death, it is not considered a gift, and it is a prohibited bequest to an heir. In this case, the Zakat is due on your father. You must withdraw the money from the usurious bank immediately and dispose of the usurious interest by giving it to the poor. Unlawful money is not subject to Zakat; rather, it must be disposed of entirely. Your father should allow you to manage the money freely, especially in helping your indebted husband.