K.W.
Unfortunately, I know what you mean. Sometimes separation is good to help them understand what they are missing. I hated it, but there also comes a time when the kids need to realize that their grandparents are doing. You can't sheild them from that forever.
My in laws were like this when our girls were young. We had to sort of cut them off for a while. After a couple of years of that, we began to rebuild the relationships and they became much more interested in my girls because they realized how much they were missing.
It's tough, I know. But, sometimes family relationships are the hardest and hurt the most. You may just have to teach your kids about this sad truth a little early!