Okay--they're old enough to know about the move next summer; I imagine that being a military family, they're probably more accustomed to moves than most. But I'd try fairly hard to nail down what's going to be happening in the meantime and tell them; it's the unknown that is scariest.
If you choke up, that's okay. My D12's seen me cry quite a few times during this time. Crying lets them know you're not just tossing your marriage out like a piece of used tissue; you're sad, it's natural to cry. If you become hysterical, remove yourself. Otherwise, don't worry too much about it.
Yeah, marriage vows being sacred--that's a tough one. Because really, you can't talk negatively about your H to them. You could simply stop short after saying that yes, you do still consider marriage vows to be sacred. Then bite your tongue.
If they ask questions, answer them as honestly as you feel appropriate. If they feel that you're not being honest they'll stop asking, and fill in the blanks themselves. And that's not helpful. You don't necessarily have to tell them details, or tell them the whole truth. Stick to the basic guidelines of it's not their fault, there's nothing they could have done to change it, they're going to be safe and loved, and maybe add that to some extent you're all going to have to make it up as you go along and don't yet have all the answers to how it will be. But you are still their parents and will still take care of them no matter what.
The only ways you can really blow it is to allow them to accept blame, to trash their father, to lean on them for support or expect them to parent you, or to lie blatantly.
My D figured out that H "has a girlfriend" on her own, because of how he acted, protected his cell phone and computer, and had numerous lengthy calls during D's visits. I wouldn't have told her. But when she asked me point-blank, I couldn't deny it. She had even figured out who it was, because H had talked about her--just a bit too much. I didn't elaborate, I simply said, yes he does have a girlfriend. Once she knew the truth, she vented her anger a lot, and then she was fine (as in not terribly traumatized). It still makes her angry at times, she still wonders "why did he get married anyway if he wasn't going to take the vows seriously?" But she also needs to have an okay relationship with her dad, and so--since she is able to ask me questions and trust the answers--she just stuffs that down while she's with him. She says someday she will ask him herself why he's done this, but it will be a long time I think--she doesn't want to jeopardize their relationship, and besides, he has declined to answer some of her questions. "I don't want to talk about that right now." And she understands why, but she doesn't push. She has been far more resilient than I anticipated, and while it breaks my heart to see her go through this, I no longer fear that she's going to end up an emotional cripple.
This is definitely one of the hardest conversations you'll ever have in your life. I'm so sorry you have to do it--that any of us have to have this conversation with our children.
M60 H52 D20 M14 yrs OW-old gf from 1986 bomb-5/18/08 H filed for D-9/10/08 D final 4/24/09 xH remarried (not OW) 2012