Shunning a child for marrying a non-jw is not in itself a df'ing offense. I did that and I was not df'd but a marking tlak was given (no names given) about doing that. The parents must have carried it one step farther on their own to making it like a df offense.
As to grandchildren, "necessary family business" has been used by many elders I know to justify associating with df'd children. The rules are always changing.
*** w83 1/1 p. 31 Questions From Readers ***
Another sort of loss may be felt by loyal Christian grandparents whose children have been disfellowshipped. They may have been accustomed to visiting regularly with their children, giving them occasion to enjoy their grandchildren. Now the parents are disfellowshipped because of rejecting Jehovah’s standards and ways. So things are not the same in the family. Of course, the grandparents have to determine if some necessary family matters require limited contact with the disfellowshipped children. And they might sometimes have the grandchildren visit them. How sad, though, that by their unchristian course the children interfere with the normal pleasure that such grandparents enjoyed
*** w74 8/1 p. 471 par. 21 Maintaining a Balanced Viewpoint Toward Disfellowshiped Ones ***
What the elders are concerned with is that “leaven” is not reintroduced into the congregation through spiritual fellowshiping with those who had to be removed as such “leaven.” Thus, if a disfellowshiped parent goes to visit a son or daughter or to see grandchildren and is allowed to enter the Christian home, this is not the concern of the elders. Such a one has a natural right to visit his blood relatives and his offspring. Similarly, when sons or daughters render honor to a parent, though disfellowshiped, by calling to see how such a one’s physical health is or what needs he or she may have, this act in itself is not a spiritual fellowshipping.