Hello lovely :) Bit late to this one but hope you managed to get it sorted.
My first thought was how would the children feel respectively, I guess I maybe see it as more of a time for the kids and not sure how it would be for them to have mum disappear off with a new partner while some are with their dad and the others who aren't yours are either without either parent or separated from their siblings to go to their father.
I'm not saying they wouldn't have a great Christmas with you, I'm know they'd be wonderfully looked after and have a fab time with you, just struck me when I read that how they'd feel, however well adjusted.
Does the new partner have kids himself? However well meaning, taking her away from her kids at Christmas, especially while some of the kids are so young, and being such a new partner, if it were me, as a mother (hypothetically speaking) I'd definitely want a say in that.
Could they not go away for a break and a rest sometime after Christmas and the tour?