Must children pay zakat from their own money if their father delays paying it on their behalf, knowing that he pays it as a donation from his own money?

It is not permissible to delay the payment of Zakat (definition) after it becomes due without a valid excuse. One who delays it incurs sin and the owner of the wealth bears the responsibility for this delay, because Zakat is obligatory to be paid immediately. The children's obligation is not discharged merely by their father's commitment to pay it on their behalf, but rather by the actual payment of it to its rightful recipients. Therefore, if a child knows that his father has not fulfilled his commitment to pay Zakat on his behalf, he must promptly pay the Zakat.