Meet at a park, bring a snack (so there's no question of sitting down for a meal but hungry kids can get their needs met) be in charge of yourselves (you, husband, kids) in the situation...that is all you can do. Meeting at a park means that it's neutral ground, you can leave immediately if your MIL starts to pick a fight, etc.
The whole thing is very sad when there is such a high level of dysfunction going on. This may be emotionally pretty painful for your husband, so be aware that no matter what happens, you may both come away from the encounter with conflicted feelings. This would happen with me when I used to visit my toxic mother when my niece and nephew were staying with her-- she wasn't drunk, but has untreated borderline personality disorder and it was horrible to watch her interact with them. There was no approaching her on her behavior, no remedy one could suggest which would be heard without her going on the offensive. She always felt that her complaints and criticisms of others (which were terrible, the kids were little and she just ridiculed them for what they couldn't do instead of enjoying *who* they were)--those hard words always were 'justified'.
All I can tell you is, hang in there with this one 'last' visit, make a plan with your husband that allows your family to leave without a big confrontation in front of the kids (if at all possible) and do your best. It sounds like, when you move, you'll have more space (emotionally) and be in a better position.
For what it's worth, unhealthy people usually marry other unhealthy people. There is a LOT of codependency going on in your FIL/MIL's relationship, but the more you point it out, the more of a 'unified force' they will be. I dealt with this for a few years where my own mom would complain of her fourth husband being 'violent' and such, and as many times as I offered her a place to stay, she wouldn't ever leave him and when he would agree to go to counseling, she'd let him off the hook. "Just the fact that he's willing is enough". The real truth was that counseling was scary, because it would have required both of them to face up to their mutual dysfunctions. They apparently (I have no contact with them) stayed together for another 10 years or so after that before she found someone else and, no surprise, left #4 for #5. All that to say, we have to let other people's 'crazy' be just that, theirs. My life is 100% more pleasant without my own mom in it. Enjoy the space when you move!