This whole thread has turned into which parent has the last say, which parent's decision trumps the other parents decision.
If it doesn't harm the DC to spend every goddamn Christmas with one parent, why should that be the mother??
I hear posters saying 'Well, she spends the majority of the time with the DC, so she should spend Christmas with them too'
You could look at it the other way....mum gets the majority of the time, so why can't she share at Christmas?
What has always concerned me about your posts karma is your justification for your arguments, good parent/bad parent labels.
Children pick things up...other posters have said this too. Your posts insinuate children are nothing more than a commodity or a prized possession of the mother, with whom she will share with the XP if she feels like it??? The children are not the belongings of the mother!!! 
To view them as such, and to treat them accordingly is wrong...full stop!!
Even a family court will grant alternate Christmases to seperated parents.
The dynamics of sharing Christmas or not are not what concerns me in your case..it is your reasons for doing so:
If she left him, then it seems unfair that she also gets to deprive him of every Christmas with his dd. Otoh, if he left her, then it would seem unfair to me to also leave her without her dd on Christmas day. If it was a mutual, falling out of love and no ones 'fault', then I think it would be fair to alternate. This is clearly saying that the children's routine at Christmas is based upon who did what wrong in the relationship and who was at fault!! The childrens Christmases would be totally different had the parents mutually agreed to split. THIS SHOULD NOT BE OF ANY CONCERN TO CHILDREN!!!
if there is an innocent party in a break up, it seems wrong and cruel to me to make them be without their child at Christmas, when it wasn't their behaviour which caused the relationship to end. Putting the parents 1st and foremost again, most parents who have split up ime put their DC first....not themselves (unless they are particularly bitter...no good for the DC either)
where a child is too little to have an opinion, then if one parent is going to miss out, I don't think it should be the one who would have stayed in the marriage, but for the behaviour of the other person. Again, coming back to who is to blame...let's label parents bad cop/good cop...again no good for DC. Also, if DC are too young to speak, that doesn't mean you can conveniently choose not to do what's right for them, based on the fact that they couldn't tell you. That's just weird! There are many instances where parents do what is right for their children without waiting for their children to verbalise their feelings, and I'm sure you must be aware that much of the time, even children who can verbalise their feelings do not want what is good for them.
I'm sad for you that you were put in that position in the first place and think that your ex, if he had anything decent about him at all, wouldn't have taken your child from you at Christmas, given that he was the one who behaved badly. So if a parent behaves badly, he is now no longer a decent parent...
. It is their child...not just hers.
I must admit, that when a person has an affair and leaves the spouse and dc for another person, I find it very hard to think of them as a decent person or as a good parent. I am strongly of the belief that good parents don't cheat on their children's other parent. Again...good cop/bad cop... A partner who cheats can, and quite often is still a good parent. Parent and Partner are not the same thing. You do know this don't you?
It is a selfish attitude wrt the ex partner. But if my husband had an affair and left me, I'd not be inclined to care about what he wanted. It's not about what he wants, it's about what is best for the DC!!!
I did say that the person who cheats/behaves so badly that the relationship cannot survive is the one who should miss out on getting the kids for Christmas, ...He should miss out because he behaved badly towards his wife??? That is punishment, as hard as I try, I can't read that any other way.
I'm not saying that people are wrong to leave relationships, only that if you behave badly, you should be the one to lose out, not the person who didn't. Again, reiterating what you have already said...justifying punishing the parent who dared to upset the mother...making her the more powerful parent.
It's not using the dc to punish the ex - it's not considering the ex's wishes and it is putting your own wishes above those of the ex, that is true It is using the DC to punish the ex, in fact you are only considering the ex and how your actions will affect him, not thinking of your DC.
The hours of labour with no painkillers, the bleeding nipples from bf, the endless sleepless nights, and the not fully recovered vagina give me the edge I feel exotic wink ha ha ha ha ha..I see this was a joke, because no right minded mother would think this way ha ha ha
I stand by my view that when someone has an affair, they stop thinking about their children's stability and security and what is good for them. For the duration of the affair, they become utterly selfish and focussed only on their own feelings and desires, to the exclusion of everyone else's. For me, that is completely incompatible with being a good parent. A good parent wouldn't put their dc in this position.
If you believe that a parent who cheats on their partner are a bad parent, please let me remind you that there are countless ways of being a bad parent, countless ways...and tbh, how can concentrating on your ex and his feelings (as shown above) to the exclusion of your DC's feelings be seen to be selfless??
You appear to spend alot of energy and time thinking about what your ex deserves, what your ex did, what your ex should miss out on...virtually every one of your posts has detailed some reference to the ex's bad behaviour towards his ex partner, not his children and how you feel he should pay for these misdemeanors in the past relationship...yet I have repeatedly asked what about the children, what do they deserve, and received replies like ' If the children are too young, then you can please yourself (which would be punishing the ex more with his children I gather?)
Instead of waiting until your DC are old enough to realise there is an alternative to spending Christmas with their mother every year...why don't you ask them if they'd like to spend Christmas with their father if he is happy to have them on Christmas day?? Frightened of the answer??
One day, you will realise that you cannot use your DC to punish other people, or stave off loneliness/hurt/pain. One day, they may ask you why you were so selfish. I hope you don't answer with 'Well your father was blah blah blah blah blah blah and he didn't deserve it/I was the wronged party' This will only serve to confirm to your DC that during their childhood, it was your feelings that mattered more!!
(In case your wondering...the right answer would be...because I felt it was the best thing for you DC!! I thought I should point that out, you may find it helpful.)