Happygran1964 - I was bemoaning the fact that the piles of plastic bags filled to the brim with presents, with a nominated place for the children to find them, was over now that they are older....I dread to think what your DS would have made of that. His reaction seems to go along with the unusual parenting ideas, latter day hippy type.
I have 7 grandchildren, who have ages ranging from 26 down to 10..I gave the older ones gift vouchers but with small stocking fillers (maybe more for me than them) to open on Christmas morning. The younger ones I had spent exactly the same amount of money on each, but never in my time have they been rationed to the amount of gifts I have been allowed to give.
If your DS and his wife feel so strongly I suggest that they do not give presents ever again but donate the equivalent amount of money to a charity chosen by the children....my 11 year granddaughter did this at Christmas, so she had no presents at all, but made a donation to a charity for the homeless, and all this whilst her sister opened her presents, I was amazed but extremely proud, so maybe this approach might help