Why can I not give my grandchildren presents?

You can't change it if that is what your son wants, so I'd try to embrace the new way of doing things instead. I understand that this will be hard but you are probably more resilient than you realise! It has obviously worked just fine in his partner's family so try to go with it and save yourself some heartache ?.

The children are too young at 2 and 7 months to differentiate between gifts from you and gifts from Santa.

Do you mean that your grandchildren will be opening presents at your house, from under your tree, as well as the presents Santa leaves at their houses? Our very young children believe Father Christmas goes to grannie's as well.

Really it is up to your son and his wife how they do Christmas. Grandparents support parents and something like this isn't worth conflict.

At the age they are now, your Gdcs won’t be remotely bothered, but I do sympathise - I’d feel the same.

I don’t understand this ‘everything from Santa’ business. In our family (and many others from all I gather) it’s always been little things in stockings only from Santa, other presents from family or anyone else are labelled as such and the givers will be thanked.