Yammy
Your child, your rules, you choose when they see grandchildren, they call their grandparents what you want them to. Visits are agreed upon by both sides.
We moved away for the same reason as you, so far arrangements had to be made therefore no unexpected visits.
I had a stifling MIL whom I wanted to love and would have been but for her behaviour which she rued later on in life when she had no husband and needed family.
I was too young to set my own rules at first until one of mine had a nasty accident when she let them do something while I was out of the room I had always banned. They still carry the scars to this day.
Yes, your child, your rules, but IMO that does not extend to dictating what the children call their grandparent, if the grandparent object to it.
To me, that one sentence devalues all the other perfectly valid statements about parents rights over their children.
'*Nobody*' has the right to dictate what another adult should be called, certainly not their own adult children/in-law!
Those adult children have the right to decide how both they and their children are addressed, but not how to address anyone else against that person's will.