I'm not sure whether the grandkids all live in the same area or not, but an idea might be to give the family zoo or museum memberships and go together at least once. Perhaps they could still give a small gift to the kids.
Or, we started including my grandparents in the exchange with the adults so they weren't buying for so many people.
We've also done where the grandparents give a gift card to Build A Bear for one age group and the parents chip in any add'l cost for the outing, and the group goes, an older group could go to a Lego store or where there are activities and for others a basketball or hockey game.
My great grandmother gives me $$ for my kids and then we buy the gifts for her to give them (the kids do not know this) and at the gathering she attends we bring them for the kids to open....just in case the effort is a challenge for your parents too.
AE