Raising grandchildren while their parents are abroad. How much discipline is mine to give?

My son and daughter-in-law are in Dubai for three years for work. My grandchildren, 8 and 5, are with me and my husband in Coimbatore. We see their parents on video every evening. The difficulty is that the children know that I am not their mother. When I say no to something, the older one says "I will ask amma" and then amma says yes on the video call without knowing the full story. I do not want to complain to my son and become the villain. Grandparents who have done this, how did you settle the question of who decides? I was strict with my own children. I do not know how strict I am allowed to be with theirs.