I've looked into going away at Christmas, my youngest is 19 and she is coming with me.
To get somewhere cheap you have to book by February (unless 1k is cheap for you), unless you are thinking of still being in the UK.
Depending on where you are flying from, flights can be limited.
My Mother didn't care about Christmas, or me, enough to fake it, I've always resented her for it.
They're going to want a Christmas with you, they will be having one with their Dad, so it's very different to just changing your family tradition.
Be prepared for them to not forgive you for going away and for not having a Christmas with them, even if it's the week before, or after.
""The way some kids have parents of different religions and celebrate different holidays. ""
But they would have their version of Christmas.
They are way too young to just not do Christmas.