You should calculate the money within your possession and pay Zakah upon the passage of a Hawl from the time of this calculation

A: Zakah is due on the money of both the grandmother and her grandson for each of the past years. It is Wajib (obligatory) to pay one-quarter of one-tenth for each year, as well as to repent to Allah (Glorified be He) for the delay in paying the Zakah.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.