I am so sorry J.. I am a single mom of two kids. I know how hard all of this is on top of wanting not to have your little ones hurt. This decision is such a hard one. You realize it impacts them and I can only shed light on what my experience was.
My kids were young, almost one and four when my ex left. I assumed they didn't need a lot of information so I didn't. It backfired.
I had to finally take my daughter to a play therapist as she had so many questions and built up anger because of the situation. She and I had to relearn how to talk about this stuff. My son now 4 is starting to ask questions.
I learned early on, be honest, at whatever level they are mentally, be very honest. Allow for questions and answer them.
Sit down with them and do it with your husband so they realize they are not losing you both, that the situation in the home will change. Your oldest is on a whole different place mentally and maybe you can talk to the oldest alone as they will have different questions and more able to handle the truth.
After learning to communicate what was happening in counselling to my daughter, I explained it like this "mommy and daddy just cannot figure out a way to agree on very important things. By not agreeing on this stuff, we are both very unhappy and it will not be a happy thing for any of us if we try and stay together". "in order to be good mommies and daddies, it is important we are happy" "we both love you and will always be mommy and daddy and be here if you need anything", "we just won't be living together in the same house anymore" "this has NOTHING to do with you, it is about us and not being able to resolve this problem". Constant reassurance to them not being to blame, keeping things as structured and normal as far as rules and routine is VERY IMPORTANT!! They need to know their world will change but not completely. Keep everything on the norm as far as meals, bedtimes, rules and all of that.
Granted I had the added stress of dealing with my ex moving completely 1,000 miles away. So it made everything more difficult for them to understand. If your ex is moving to a new place, have them see it and have them have a place of their own there. With never hearing you put down each other, that will help them accept this situation. I have to bite my lip constantly, I am not duty 24/7, however I have taken the high road for them. I foster the relationship with their father, though 1,000 miles away, I make sure they understand I want them to know how much their dad loves them.
I encourage them calling him, staying in touch. Though now realizing it isn't my job as they get older, he needs to fail or succeed as a father himself. I tried too hard to protect my kids and figured out the hard way, they will make up what is going on in their own minds and it is typically way worse, so being honest is the best way to go.
Good luck, God bless and if you need moral support, I understand completely, just side note me.
Hang in there! :)