I have 3 male cousins that just left off association with the WTS when they turned 18. They married nice, loving non-jw women and had a family. They did not live "nasty" lives, overdrinking, smoking, swearing, etc. They are good husbands, fathers, workers, friends, sons.
Despite the concern of their wives, the parents came to a joint decision to let the jw parents have unsupervised association with their children as long as the jw grandparents promised not to preach to the children. The jw grandparents agreed.....not!
One wife found out that grandma had been reading the bible story book to all the kids despite the "promise." So she lied. All 3 sets of parents called a meeting with their parents and told them there would no longer be any unsupervised visits with their children.
Now remember, none of my cousins were da'd or df'd, their parents could have visited their grandchildren with the parents there; but their intent was to convert the grandchildren behind the backs of their parents.
I have seen this done over and over in other families.
My suggestion is that if they want to see their grandchildren, their parents have to be there, inactive, df'd, or da'd.