I used to do baking with DD from before she turned 1 (she could mix together the flour and spices and baking powder with a spoon that I also held, and use a fork to mix the eggs in a bowl/jug - and it built from there to she now wanders in with her phone having found a recipe online, finds all the ingredients, makes an almighty mess, but sometimes turns out nice things!!
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But she always baked cookies for Santa on Christmas Eve. We use the same basic cookie recipe, that you make a log of dough and slice off the rounds of cookies. It can be made ahead of time and kept in the fridge or frozen, so I always make some in earlier December and freeze half the batch in case we don't have the time or energy to bake from scratch on 24th. But they are relatively straightforward to do if we have managed to not be too busy and want to start with flour, butter, sugar and eggs.... DH and I are normally doing things like peeling veg and other jobs for the turkey feast, together, and DD generally does some helping out before wandering off to entertain herself.
We also have a family tradition that is somewhat rooted in Irish tradition - that once it's dark (we do it after our early dinner), the youngest in the family lights a red candle that is then placed in the window to show any weary travellers on the road that there is room for them in our "Inn". Our candle goes on the mantlepiece over the fire after DD has lit it, but before it is moved, we have a short interlude as a family where we remember the good and bad things about the year just finishing, and to think about people who have died and cannot enjoy our celebrations (whether immediate family we would have seen, or further extended family or friends who are missing their own family celebrations). We finish with a "Hail Mary" prayer (the tradition would be a decade or even the full rosary - but we are not particularly religious and this is the level that feels right to us).
Then the Hamper comes out with new PJs and a bath bomb/shower gel for everyone and DD's stocking etc (the Hamper Elf is, and always has been, me, and fully acknowledged as such - so reusing things every year is no issue, and occasionally I get suggestions from 1 for the other about their new things). So DD organises the stocking and Santa's snack (including cookies), before heading for a seasonal bath, new pjs, and a hot chocolate together before snuggling into bed (clutching her snowman covered hot water bottle that gets used all winter, and listening to "Twas the Night before Christmas"). Although, she gave up on the bedtime story last Christmas (aged 13) - but still insisted on everything else!! 