Cute idea, though I’ve only heard about Christmas Eve boxes. I guess I’d suggest picture books with a Christmas theme (I imagine your kids are reasonably young?), perhaps some Christmas themed craft kits, stationery to write a letter to Santa, maybe a biscuit mix to bake biscuits to leave out for Santa.

Very cute idea to make a 1st December box. I’ve already bought our family matching Christmas jammies, so I would start with that. Maybe a Christmas movie you can all watch together to get the Christmas season started. Maybe a movie night pack with some snacks everyone loves. Then it becomes a whole event for the house

Oh how cute! I didn’t even know this was a thing! But I love it! I know my sister does elf on the shelf and always buys new Christmas pjs when they start. In our house when we put up our decorations I would write down Christmas movies on slips of paper put them into a tree ornament we have. Then whenever we want to watch a movie we take turns in drawing out a movie. It’s always been fun.
I look forward to reading what people recommend.