It would appear this is a moot point since you said there was an agreement in place already to photograph the kids.
This may be an opportunity to parade your creative skills – come back with a suggestion of a holiday theme that is not ‘cliche.’ I’m sure you can think of something!
I believe one of the qualities of a good photographer is his / her ability to apply fashionable (timeless) ideas to a trendy situation. For example, my own high school graduation photos were done in the early 80’s when big hair was ‘in’ for girls and gigantic bow-ties for boys (in tuxedos). But our photos in my yearbook are STILL good today because the photographer had the skills to pose everyone properly so your eyes are not drawn to anywhere else but the faces.