It sounds like you are in a very difficult situation. I am sorry I don't have much advise. I can see in a way your damed if you do, damed if you don't. If you do distant yourself from them to protect yourself perhaps then you might regret it later if they decide to cut you out of your grandchildren's lives. But if you don't they will continue to treat you the way they are.
As a teenager I couldn't stand my mother. I was rude to her (verbally) and I honestly went through a stage where I hated her. As soon as I had my own child (when I was 17) I suddenly realised what a Bi(a)tch I was to her and over the past few years our relationship has improved and we now have a fantastic mother-daughter relationship. It is all water under the bridge now. My eldest is very challenging and I can see us having a huge difficulties ahead especially as she is older and going through puberty. I can only hope that once she is on the other side and an adult things will improve.
I hope you find a solution thats good for you and improves your relationship with your children. I haven't had much luck with professional family therapists in the past but maybe some advise from people that have experience dealing with family problems would have some good suggestions.
Good luck