In the kindest possible way, you're over thinking it, honestly! Or, perhaps, thinking like a grown-up!
The kids don't need to receive everything on their lists from Santa. (DS wrote he wanted a phone, computer and an iPad. No way is that going to be what he gets in his stocking!)
DS is 7. We've spent every other year with the ILs or my family. Father Christmas behaves completely differently in each house and DS has never noticed - or cared enough to mention it. In one house, Santa brings little presents in stockings on the beds, but nothing under the tree. In the other house, Santa brings all manner of presents in the stocking and under the tree.
There is so much going on - gazillions of present,s toys to play with, lego to build, food to eat, grandparents to see etc etc, a minor detail about who gave him wat really is irrelevant.
The magic of Santa is that he comes. The presents are a part of Christmas in general, the DS will be thrilled no matter where they came from.
If by any chance, they do say "Santa didn't get me everything" just say something non-committal. Then when they get the presents from your parents, say "clever Sants, he must have known your GPs were going to get you those presents. Only say this if they question it themselves though - and they almost certainly won't.
Just have a lovely day, enjoy the magic of Santa, and DC spending time with grandparents, and stop thinking like a grown-up!