keep the money of the orphans till they become legally competent. Then, you should give it to them in person or through anyone you trust. If you increase it through legal methods for their sake, it will be better, as Allah (Exalted be He) says:
And come not near to the orphan’s property, except to improve it, until he (or she) attains the age of full strength
You should pay Zakah for it every year because you are their guardian and deputy. It is Haram (prohibited) to borrow from it and add it to your money for your sake. At the same time, Zakah should be paid for their money. Thus, you should restore and maintain this money till they become legally competent.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family and Companions.