@IsThisOneFree
Well, technically going NC avoids the 'showdown'. Because you 'just do it'. You don't call and say "I'm going NC and this is why". You just block, delete, spam-file their emails. There is no discussion, explanation, or reasons given.
So, based on your 2nd & 3rd paragraphs, I'm sorry but I don't see how you'll be able to go NC and yet control what they say to and/or the relationship they form with your children. If you had an 'intermediary' who could be present, maybe. But if you're truly NC that means the older will see their uncles without you present so you won't be able to control what is said, and your younger will not be able to see them at all, since NC means you wouldn't be able to be there.
I'm going to try to put this in a kind way, but why do their opinions mean anything to you or their disapproval seem to bother you so much? So they disapprove of choices you make? So what. But it seems as if their opinions have some sort of 'weight' with you, even if you disagree with them. It just seems that it 'matters' that they think well of you. Oh, I don't know if I'm saying it right. But what they think of you doesn't matter. What they want you to do doesn't matter. And you need to be able to get to that 'place' within yourself.
And I hope I'm not offensive, but is there some sort of cultural patriarchy at play here? I'm just trying to put myself in your position to figure out why you're willing to turn your life inside out to 'silence' them by NC or disappearing when ignoring them will do. In your position, if my brothers in law tried to run my life after the death of my husband or were going to speak ill of me to my children, pardon my language but they'd get a hearty laugh and a 'fuck off with your opinions' from me. And not taking 'no' for an answer? They'd get the phone put down on them. And they would never see my children again. If they showed up at my door, I would call the police.
If it turns out that 'disappearing' or going NC isn't going to work for you, google 'grey rock'. And consider counseling to build up your courage and your belief in yourself. Seriously, you are the 'captain of your ship' and they don't own you. An old proverb says "The dogs may bark, but the caravan passes on". Let them continue to bark, your caravan can ignore them and keep moving.
And if by not knowing their reaction 'that you can't predict' means that you fear violence or verbal abuse, remember that it is illegal for them to harass or persecute you and there are legal ways to prevent that. The police and the courts can get involved to keep them away from you and from your minor children.
And also they have no 'right' to a relationship with your children, especially if that relationship undermines you, their mother. And your children shouldn't have a relationship with them for the very same reason. You don't permit things for your children that will be detrimental to them in the long run and having their uncles run their mother's life choices and her reputation into the dirt IS detrimental to your children. Protect them by keeping them away from these people.
Don't live in fear. Get what help you need to build up your courage and to keep them away from you and your minor children.
And please tell me that none of these people have a key to your home. If so, get your locks changed.