I think you should not be around your grown kids, regardless of if they try to reconnect with you, until and unless something changes drastically here.
Bullying you, using you for money, stealing from you, getting violent, wow. They sound completely out of control and dangerous. This could get a whole lot worse. Do they have any substance abuse issues, by any chance?
I will say that I have cut off some family members after many years of unrelenting, unexplicable shabby treatment and I would not want my child around them. In my case, it is not from a desire for control or to punish them but just because I didn't want my kid in that mess, without me around to monitor the craziness. Also, it's not really possible to have a problem person out of your life if they're still in contact with your child. I mean, obviously in my case, I do not think they are a positive influence in my child's life. (My child is grown now though so this is all past tense).
Sorry, I digress. But in THIS situation, I would be concerned about the grandchildren's safety with these parents. But here in the US anyway, I think you have to have actually recently seen something yourself to get an investigation going. So it would probably be for someone else to report if they see anything.
So sorry you're in this situation but I think you are definitely better off without them and hope you don't ever allow them back into your life without seeing proof of serious changes. Safety first!
Human relationships get so hard. If I didn't have my lovely kid and lovely husband, I think I'd just get a couple dozen cats and call it a day. 