I agree that Christmas isn't "for the children" - it's also a religious festival - but as the majority of people don't see it that way, then the 'its for the kids' attitude makes more sense.
I remember as a child Christmas not being much fun because we'd open pressies, then have to get ready to go to Church (which to be fair, I always enjoyed on Christmas day) then just as we'd got back and started playing with our new toys we'd have to get in the car and go to my grandmother's house (always my mum's mum, she felt that DDs should go to their Parents and so what if mum's MIL only had sons? Thankfully, my uncle spent Christmas with other granny). Then there'd be a long wait for the meal, so we'd fill up on crisps and snacks, then not want it, we'd have to sit for what felt like hours while all the grown ups enjoyed endless courses. TV on Christmas day was 'common' so banned even if there was a great film on, however the evening tended to be rather fun as we'd play 'palour games' with cousins, but we normally had to go home relatively early as we were exhausted.
Boxing day was the fun day, the day we actually looked forward too - we'd get up and play with our new toys in our PJs and eat bacon sarnies (only time of year we were allowed to eat breakfast in the living room), then get changed about 11 when my dad went to fetch his mum, we'd have meal close to a normal sunday roast, it would far shorter (as no one would want a huge feast), we'd then play with our toys /watch what was on the TV.
The best year was the year my mum's mum fell out with my aunt and uncle so wasn't seeing them at Christmas, so she came to us instead, Mum and Dad were able to enforce their relaxed 'children focussed' christmas - my Nana was rather 'cats bum' about it all (not a posh meal, TV on if we wanted it, lots of toys all over the living room).
so while it was good for my Grandmother to have us all there, it meant that christmas day was (all but one year) a bit crap throughout my childhood and my parents weren't able to do the Christmas they wanted or build their own traditions. If they'd insisted on their own unit one year, I bet we'd have had a lot more fun.