Hi there! Sorry I'm late answering your question. I can hear how tortured you are, over this. That only makes you a normal person, let me reassure you - emotional programming notwithstanding. There are some things you can do, to help yourself and most importantly for your kiddos. (That said - I second Hops' question and hope that you're seeking 3rd party feedback and help to learn even better ways, that I can suggest, to manage the situation, your feelings and learn some "difficult people" skills.)
So, on to the suggestions:
1. Do not allow yourself to wonder, fret, or otherwise be concerned by the actions of your parents, in enlisting others and brainwashing those people, to think you are some at-fault, ingrate son. It's none of the other people's business, is it? They don't know the whole truth anyway, and their opinion doesn't "count". Furthermore: no action you can take, no revelation or enlightenment on your part is able to adjust the frequency/character/static of your parent's feelings, actions or treatment of you. Sometimes, it helps to resort to strange "tricks" on ourselves to get through the kind of highly unpredictable scenario you're worrying about. (Remember, it IS in the future so you really don't know how you'll feel or what will happen then... it might turn out completely differently than you fear it will.) Difficult as it sounds, it might be possible - if you're feeling very uncomfortable in that situation when it actually happens - to imagine a force field around yourself that either makes you invisible to the parents... or makes them invisible to you. Inside the force field... you consciously breathe very slowly, take very careful notice of all the other "reality" and people around you... and walk yourself through basic body relaxation exercises. And make sure you are paying attention to your kids' performance - that's why you're there!! This isn't some showdown at the OK corral, you know?
I have one more comment on this suggestion. Part of the reason you're feeling so freaked out, I think (correct me if I'm wrong), is that you are directing all your attention to the parents and what they are doing - and what they MIGHT do. As long as you allow them to dominate your thoughts... they are in control of you. That's not what you want and it's counterproductive to being able to live your life and feel comfortable in it - no matter how insane the parents are. You need to focus on you, finding a way to soothe yourself and generate some confidence in your ability to "get through" any interaction the parents try to force upon you. With civility and self-respect. So that your kiddos can be proud of the way you handle yourself. You're demonstrating to the world - and those important people in it - that you aren't like the parents. Got it? No more imaginary conversations, confrontations, embarassing possibilities in your head. Your goal is simply to be on your best behavior - forget aggression, hostility, revenge. That all by itself, will lower the worry and anxiety levels. Remember to breathe.......... and be engaged in YOUR life - theirs is irrelevant to you, until you are ready to deal with it piece by piece - hopefully with a good therapist and from the perspective of what you can do to heal ('coz there's zip, zilch, nada you can do about them).
2. The kiddos are way more important than your parents and what's going on between you and them. Way more important. They are "little people" and are smart about and insightful about "what's going on". Don't ever pretend to them, if they mention it, that everything's just fine - that'll insult them. They are also not "pawns" in some kind of strategic game; nor are they the "treasure" to be won or lost. They are PEOPLE with their own intuitions, feelings, and thoughts/opinions. I don't know their ages, but don't underestimate even the youngest one's ability to figure out what's going on. Be very, very careful of the kiddos right now. You want to prevent them from feeling that they are responsible for the bru-ha-ha... or that somehow it's their fault, or there's something they could - "should" - do to make the situation comfortable. I can tell you that even 4-5 yr olds will be able to detect and feel and be affected by tension, anger, fear in situations. You can refuse to take the bait and prep yourself to relax and be civil - which will lower the tension levels floating around the kids. Do not make them responsible for choosing sides. Do not make them responsible for choosing sides. Do not... well, OK... I said that, already. Don't even mention it.
Your controlled behavior, and calm, civil, rational demeanor is what the kiddos need going into, during, and after a situation like you're afraid MIGHT happen. If you can focus on that and give it some thought and energy... feed it until you actually FEEL that way... I believe that you'll be absolutely fine, even if they do show up and attempt to interact with you or the kiddos. Better to let the kiddos make up their own minds about Grandma & Grandpa - than to encourage them to agree with your feelings about them.
One other thing I do, if I start worrying about something like this way in advance of the potential situation... is that I will allow myself 15 minutes prior to event, whatever... to think about it, make plans for extracting myself, and breathe myself calm... and since I've made this "appointment" with myself - it doesn't have to occupy my every waking (or dreaming) thought between now and then.
Which comes from my own experience in similar situations... as one of the kiddos; later as a parent. I hope this helps. Maybe someone else has some better ideas - I had to invent my own survival strategies, and they're not without "side effects" or flaws.