Is Zakat obligatory on a sum of money deposited for a minor daughter in an Islamic bank, given that a [hawl] year has passed over it, and knowing that the principal amount is decreasing?

Zakat is obligatory on the minor's wealth if it reaches the nisab (minimum threshold) and a hawl (lunar year) passes over it, regardless of whether the minor has reached adulthood or not. This is the view of the majority of scholars and the saying of the Companions and the Tabi'un (followers), due to the generality of the Islamic proofs, such as the Almighty's saying: "Take, [O, Muhammad], from their wealth a charity by which you purify them and cause them increase," and the Prophet's (peace be upon him) saying: "Inform them that Allah has enjoined upon them a charity to be taken from their rich and given to their poor." The guardian is responsible for paying Zakat on behalf of the minor's wealth. It is recommended to invest the orphan's wealth so that Zakat does not diminish it, as was reported from Umar ibn al-Khattab (may Allah be pleased with him).