Ike - just read all this and I'm so sorry. This must have been a terrible shock and I suspect the pain is now kicking in.
I do understand your dilemma about Christmas. I wonder whether a part of you feels that if he is with you all at what is normally a very "warm inside" time of year, he will think: "I can't give this up"?
I also understand your worries about the children and I am torn.
When my children were little, it was terribly important to me to give them lovely Christmas memories, so we had family rituals and I even created "smells" from cooking, candles etc. that in years to come, would always remind them of this time of year.
While I agree wholeheartedly with the others, that asking him to leave before Christmas might be the best for you, I'm conflicted about the children and the memories this might give them of Christmas.
I read from what you say (and between the lines) that you are still hoping he will come to his senses. If this is what you want - don't be afraid to say that to us.
If it is, I'd suggest you go into battle mode now then and the very best way to help him change his mind is to make him believe you want out too. Do make yourself feel good with your appearance and stop telling him everything. Be cheerful but secretive in his company. Ask him now to start getting his stuff together so that moving day is easier. Tell him you're thinking of advertising for a student lodger (hunky young males only of course!
You may have seen my advice on numerous other threads like this, but I often suggest that at some point you very calmly say to him: "Actually, I realise you haven't been meeting my sexual needs for some time now. I'm really quite excited about giving someone else the opportunity to do that..."
He probably thinks he's some sort of stud muffin, having all these affairs. I'd love him to have a niggling doubt that actually, he's a bit crap in bed, but that no-one has ever had the honesty to tell him, because they've wanted other things from him.
If you want him back, he needs to see you in a different light, I'm afraid. The OW thinks she's got all the cards stacked in her favour at the moment, being understanding and waiting in the wings. You need to play a better game. Before anyone comes on here about women fighting over a prize prick - I agree, but if this is what Ike wants - and it will help her esteem at this awful time, that's fine.
Has he said why all this has happened Ike? By the ages of your children, it sounds like he wasn't prepared for the demands of parenthood, but there may be other reasons.
Be kind to yourself through all this and try to make sure you get some TLC from the people who love you most. I understand how frightening this all is and that you feel like a little girl again. Listen to that voice Ike and get some nurturing from someone who cares. If you go into battle like I suggest, you will need someone to weep and wail with out of sight of him.