I am sorry to be deaths head at this feast, but I think buying 8 presents each for your grand children is excessive. Admittedly, you do not say what they were or how much they cost and if they were all stocking presents from Poundland that would not be excessive, but I suspect you poured your all, money and love, into buying those presents and I suspect this wasn't the first time you have gone overboard like this nor was it the first time it has been a bone of contention between you and your son.
Since when have parents vetted presents bought for their children? Well, for as long as parents have been parents and children have been given presents. I can remember my mother quietly removing a present I was given that she thought was unsuitable for me, and although I do not remember doing it myself with my children I would have if the occasion had arisen.
My DS and DDiL struggle with the number of presents their children are given. They had their children late and few of their friends have children, so they get spoiled at Christmas and birthdays. As a result both sets of Grandparents liaise very closely with the parents over what we give the children, as indeed did my children's grandparents with us.It would not occur to me to give my grandchildren other than the most trivial of presents without consulting their parents.
As many a thread on Gransnet has shown, showering children and grandchildren with everything they want as a sign of love, often results in selfish and demanding children who grow into adulthood expecting to get everything they want and rejecting the givers when they cease giving.