Is it permissible for the one paying Zakat to add the value of the Zakat to his needy children to help them with marriage expenses, or must the Zakat on this money be paid (separately)?

First: The money must be withdrawn from the investment certificates, and one must repent from dealing with usury. The profits should be disposed of by spending them on charitable causes, and you are only entitled to the principal amount you invested. Second: The father's obligation to financially support his children ceases when they reach maturity and become capable of earning a living. At that point, it is permissible to give them from Zakat (definition). As long as all your children have reached marriageable age, there is no objection to giving them from Zakat what they can use to help complete their marriage.