I'm sorry you're going through this!
I can speak from the grandparent's side of things … We have one child - a son - and 3 beautiful grandkiddos. We live about 30 minutes away from each other.
We totally spoil them - I think that's what grandparents are supposed to do.
However; we understand they are not "our" kids and have to respect what their parents ask. We have had conversations about spoiling them
For birthdays we give cash that goes into their savings and a few smaller gifts.
For Christmas we give one large gift (based on ideas from their Mama of something all 3 would enjoy having) and then they get smaller gifts - gifts we give include clothing, which they can always use.
This year our son and DIL are going to find somewhere we can go as a family for 2 or 3 days instead of us giving them gifts at Christmas - they'd rather have the memories than the "stuff".
I hope you figure things out soon - maybe find a middle ground somehow … best of luck!