Christmas is all about family, and traditions.
How are we supposed to create our own family tradition when the previous generations won't let go so we can grow into our own role as Matriarch/Patriarch?
OP, you have a new house, and naturally want to establish your own family tradition. It's not like you are excluding her from joining you, just wanting to rightly have your own family christmas.
This will be the first year DH/DS and I have been in the UK since he was born.
My mum wants us to be at hers first thing in the morning, i.e stay Christmas Eve at her house, but I know that DH probably would prefer to be in our house, and I also want to have Christmas morning with my DS.
It's my first proper christmas in a proper christmas celebrating place... So I managed her expectations early on!
Next year, I dare say DH will offer to do the Xmas Dinner.. so we can invite my Mum and her DH.
The older generation has to understand that they have their time with the children, but once they are grown and have families of their own, that the new unit comes first. Why should they have it all ways?
YANBU, stick to you guns, they'll get over it in time.