Sadly, I don't think suggesting you agree to differ will work here.
There are, after all, some conflicting views than are so grave that they lead to this kind of situation.
I have never tolerated certain political views myself, and it would not have matter to me who expressed them. Admittedly if my parents had suddenly turned racists in their old age, I might have tried suggesting that we just did not discuss politics, but I would not have wanted children listening to such views.
Please, don't think I am accusing you of being racist, but obviously something you or your husband or you both said has caused your daughter and son-in-law grave concern, otherwise they would not have taken this extremely drastic step.
Please, try to move on with your life. Writing letters to the grandchildren that they can be given when they come of age is quite permissible in my book, as is an explanation of what the family quarrel actually was about. I wish my family had explained why we never were allowed to see or visit my cousins - all I ever could glean was that my uncle had offended my mother.
Try to accept that you are not likely to see your grandchildren, I know this is hard, but get help to move on - there are plenty of children in need of an "extra" pair of grandparents, so perhaps you could find a family without a grannie and grandpa to help out now and then?