"OP's grandparents care about their GCs because they took them in under their roof and did what they could (in this case rather badly) to look after them"
And said, repeatedly, that their mother "needed a slap". Seriously: if a husband said that their wife "needed a slap" MN would would be LTB in the first response. Why should the OP spend time with, and expose her children to, "grandparents" who threaten her with violence? She's the wife of their son and the mother of their grandchildren, but they threaten her with violence when she tries to deal with safety issues. In what world is that acceptable?
"GCs need to know there are other peole around who will always be there for them"
People who threaten to assault their mother when she says things they don't like. Perhaps you think children should have to be exposed to violent people who want to assault their mother, but I overall think it's a bad idea. Or is threatened abuse OK so long as you're a bit older and a bit exotic? Culture, don't you know?
"but having a different understanding of safety isn't a reason to cut a bond between relatives, surely"
But threats of violence are. Your claim is that a mother should accept being threatened with violence in order to maintain a relationship with people who are unwilling to do anything to protect her children. I think you're wrong.
"What will the GCs say years later"
They'll say "at least we don't have to see the people who want to hit mummy". I think that's a good thing. You're obviously more relaxed about assault.