We put tree and decorations up on the last weekend of November or first weekend of December (based on convenience, whether they're at mine or their Dad's). I usually let the kids decorate the windows with snow spray.
I usually decorate money envelopes and fill them for advent calendars.
In the past they had little trees in their room but no longer suitable since we moved (and got a cat!)
On Xmas Eve (or Xmas day evening if I have them for boxing day), we do a hamper with Xmas pj's, sweets, toothbrushes, toothpaste, popcorn. I used to include a Xmas dvd but now we pick a Xmas film on prime or netflix.
Xmas day we have Waitrose pastries for breakfast (they're frozen so I stick them in the oven while we're opening presents). As they're a bit older now, I'm planning to let them have a glass of prosecco too.
Not sure if you've got DC or how old, but when mine were younger we also made salt dough decorations, put out reindeer food, mince pie/carrot/alcohol on a plate, magic key on the doorstep.