I am so so sorry. I remember the feelings that you have now so vividly. My heart is breaking for you....and your kids.
In my opinion, your sons are old enough to decide what they want to do and what kind of relationship they want with their dad. I made the mistake, several times, of trying to encourage my kids to see their dad, fix things with their dad, etc. It took two counselors and my own kids for me to realize that I was jeopardizing my relationship with them by trying to help. The counselors told me that it is not my relationship to fix. Don't get involved, but don't be a barrier. It had been very hard at times these last couple of years to practice this, but it truly has been for the best.
I agree with Beatrice as far as encouraging politeness and honesty. You would expect that from your children towards any adult. This was also challenging at times because I was usually met with the response that he wasn't the same towards them. And he wasn't. Beatrice is right that it is rare that an MLCer focuses on their kids. I still shake my head in amazement about some of the things that my XH said to my kids. But you can't stop that....you can't prevent that. All you can do is be there for the kids to listen.
Unlike Beatrice, I have not talked to my kids about MLC. I can't give you a specific reason why I didn't, but I will tell you that I kept many, many things from them. I have been advised by many well meaning people to not hide as much. But I am convinced that they are the ones that would be hurt. I went through this with my parents and I was 21 at the time. I knew too much and I wish I didn't.
I think you are right in going dark because I think it is right for you.... You need this to find your footing, to be there for your kids. I can imagine right now that you are desperate to save your marriage. But please know that DBing is about saving you. It took me reading the book several times to realize that. And while it is very hard because you want to save your marriage, you need to put that on the back burner and focus on your kids and you. And, as a mother, my guess is that you don't focus on you to the degree that you should.
I know that you are probably in a state of disbelief right now. Continue to come here and post. The people on these boards are amazing and helped me so much. I also vented here instead of to XH. Look at other people's posts and comment - it will lead to more traffic and more posts to your situation...meaning more support for you.