Talk to your grandchildren and find out their favorite types of items.
Eg: Maybe they just want you to be with them (Awwwwwwwww.)
Eg: If they like electronics, buy them the latest technology (Guaranteed they don't have all of them)
Eg: If they like dolls and stuffed animals, give them a customed or limited edition. Personally, I would get like the biggest stuffed toy possible like those giant bears you see except bigger.
Eg: If they just want cash, you might as well give up on those ungrateful brats
I'm younger than 20 and I suppose your grand children are too.
Pour all your family-love potion into everything and I'm sure they will appreciate it.