LW: Your daughter is definitely old enough to decide she doesn't want to see her grandparents until and unless they make amends. (Or ever.) You have done your duty in keeping that relationship open. Those people are the ones who have burned that bridge.

You should support your daughter in whatever she chooses. If you're concerned, you can emphasize that you will support her no matter what, even if she decides she does want to talk to her grandparents again.

Also! Lots of people do not have this good a relationship with their ex! You and Kate are to be commended.