How much luggage space will you have for Christmas, rather than necessities? And would you be happy to leave things behind for anything you get there (shopping, presents etc)?
Somewhere like Poundland or Lidl might have "snowscene" stuff - white fabric cut into icicles to hang across windows, snowflake or snowmen stickers for windows/walls, etc. The fabric would be slightly bigger, but stickers will take up hardly any space in the case. Or maybe some window clings - also quite flat against the suitcase wall under the clothes.
Are there Christmas traditions where you are visiting that you can take advantage of? So celebrating the same event but differently?
Or are there likely to be decorations there that you can buy (or even hire) and just use out there?
How remote will you be? How far ahead of Christmas do you arrive? And will you be self-catering, or in a hotel?
If SC, could you do a semblance of a Christmas dinner even if not a full turkey? Maybe cook turkey steaks on the BBQ, or prawn cocktail as a starter before a lighter main than usual? Is there anything you can bring with you (smoked salmon does travel well if vacuum packed, and slides down pretty flat in a suitcase - if you are allowed bring it into the country you are visiting, for example).
Instead of Christmas jumpers, could you get Christmas tshirts instead for the DCs this year? M&S had one of Cactus's covered in Christmas lights a couple of weeks ago. And yes, Santa hats are small for packing but could make a big bang for festive impact.
And for Santa himself, perhaps anything he leaves on the day may be useful for the trip - beach toys (if near a beach), sun cream, activity books/sheets and colours, a kite....to either use up before you leave, leave behind because they are more suited to that location (get cheap versions) or pack up tiny like maybe a kite to come home. A pack of cards could be great if you can get one with a local backing. Or a board game/jigsaw to enjoy, but leave behind at the reunion for wider family to enjoy afterwards.
YYY to having a couple of seasonal movies and some music downloaded to enjoy on the trip. Maybe try to get one of those movies being one in a hotter climate - and seeing how it's different there.