That’s really difficult. Is he planning to see her on any other day over the holiday?
Given that she’s going to miss out on seeing him on Boxing day, is she more likely to cheer up surrounded by other family/cousins or having a day out just the two of you?
I think you just say daddy is poorly/has to work and will see her another day. I hate the fact that anyone should make excuses for someone’s lack of consideration but anything else will be more hurtful.