Taking care of daughter and two children

My daughter has two girls, aged 12 and 9, her husband left her and the children with no money and refuses to send any support to them. She is not working and never worked before. I am supporting her and her children until she finishes training for a job and can support herself and can get a divorce and all legal actions. My question: is the money I am spending be considered as Zakah, keeping in mind that this expense is depleting all of my resources and I am 70 years old?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

A husband is responsible for the maintenance of his wife and children if he is capable of doing so. But if the wife is disobedient and does not accept to abide by her husband's orders. Then her husband is no longer responsible for her maintenance even though he is held responsible for the maintenance of his children.
On the other hand, you are liable to take from her husband the money that you have spent on your daughter and her children. The husband is legally responsible to pay you back all what you have spent on his wife and children. However, if you have spent the money with the intention of charity then you have no right to take it back.
Know that it is not allowed to consider the expenditures you spent on your daughter and children from Zakah since a rich father or mother is responsible to spend on their poor children. The grandchildren are also like children in this ruling. Imam Ibn Qudamah is his book al-Kafi narrated from Ahmad Ibn Hanbal that he said: "Zakah is not given to the children of a daughter for the Hadith of the Prophet (Sallallahu Alaihi wa Sallam) that he said: "This son of mine is a leader" [i.e. Hasan who was his daughter's son].
When giving Zakah becomes forbidden because of close family ties spending on these close relatives becomes mandatory.
Allah knows best.