Chamonix, just say that you are very sorry but you have got a lot of people coming and cannot fit two more in. I know it is a hall but these is the polite way of saying that you don't want her to be there (which is true and TOTALLY reasonable).
Just decide that when you talk to her you will go broken record like you are speaking to a very silly little child who will not take no for an answer!
Practice in front of the mirror...
I am really sorry MIL but we are quite crowded and there is not have room for any extras...
or you could take the bit between your teeth/the bull by the horns or whatever the phrase is and just say...
I'm sorry I was a bit backed into a corner when you asked before and I said yes, but really the answer is no. We planned a Christmas with time by ourselves, time with dh's family and time with family, and that is what we want to do.
You will see us and dd on Boxing day. We will go home with your husband and dd and the next day will be a family celebration with your side of the family.
I think I would pepper my comments with things like, it's only fair you get to see us all on Boxing day and my family get our undivided attention on the day after.
PLEASE do not be pushed into this. If she says "But you said I could...."
I would be honest, whatever honest is in this situation, maybe "I felt put on the spot, I knew that I wanted to have a day with my family and I felt it was hard to say no. But having thought about it I know that it would be unfair to expect my family to share us and dd with dh's family when dh's family had already had us all over the day before, so my answer is no, sorry MIL, we will see you Boxing Day but not the day after.
Re "Should I really have said "no, it's my time with my family, you have us on Boxing Day!" (What I thought) it would've caused tears and the mother of all arguments from Fil." YES you should, you need to be honest with her. It is only fair that both sides of the family see you, your dh and your dd, and one side does not hog the whole season.
What arguments from father-in-law? With whom?
Re " had no other excuse for not staying at hers Boxing Day for the night." You had the perfect excuse, I want to go home to my own bed. That is your choice. You are an adult. Please put these boundaries in now with your in laws or they are going to make life miserable for you.