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My parents moved south when my kids were young teens and although we tried to visit them, it was hard to as the kids got jobs and were busy with school activities Their grandparents were not here to attend the events, and they were seldom here for holidays, and sent cards for birthdays. My father has passed away and my 86yr old mother moved back here to be near the kids and she has a hard time understanding why they don't spend more time with her. They do send cards.

Well, your mom is different generation and today's life speed is hard to understand for those people. (My mom past away 2 years ago at the age of 98, so I kinda understand what are you dealing with.)
On the other hand, cards can't replace the presence, and from that perspective I can understand your mother.
At the same time, no one can blame your kids either as they didn't build that more close relation with their grandparents when they were younger. And from that perspective, your mom shouldn't expect from your kids to act as it's Christmas every day (although, I wouldn't be surprised if she does).
Maybe the both sides need a little push, your kids a forward one, while your mom a backward one (to lower her expectations). I know it might take a lot of efforts, explaining and diplomacy for the one who would take the obligation for those pushes. 😊