Salam Alaikum:
Parents have strived for a relationship between grandparents and grandchildren, but the grandparents have not taken a single step to build any relationship and have no justification for this. So there is no relationship between grandparents and grandchildren, even though there is relationship between parents and granparents based on the efforts of the parents alone. The parents keep cordial and regular relationship with grandparents.
If now an adult grandchild wants to visit grandparents or send gifts to them, does a parent have the right to stop him / her, specially because there is long history of the grandparents gravely hurting the parents consistently over a long period of time; and such an action by the grandchild produces severe reaction on part of parents. The child says stopping him / her cuts off the relationship with grandparents and the parents' position is that there is no relationship in the first place to cut off.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

It is incumbent on the grandchildren to uphold the ties of kinship with their grandparents because they are among the kindred with whom it is obligatory to maintain the ties of kinship, and it is prohibited to cut them off. It is impermissible for the parents to prevent their children from upholding the ties of kinship with their grandparents; such an act constitutes enjoining evil and forbidding good, and therefore, it is prohibited to obey them in this regard. The Prophet said: “There should be no obedience to a created being in what constitutes disobedience to Allah, The Exalted.” [Ahmad] The neglect on part of the grandparents in maintaining the ties of kinship with their grandchildren does not justify cutting relations with them on part of the grandchildren, especially given that the position and status of a grandparent is like that of the parent; and therefore, it is obligatory to show dutifulness and kindness towards grandparents.

If there is some kind of misunderstanding between the parents and grandparents, the parents have no right to prevent the grandchildren from upholding the ties of kinship with their grandparents, and reconciliation should be sought. It should be noted that it is obligatory on a child to show dutifulness and kindness towards his parents, even if they wrong their child and offend him.

Allah Knows best.