When there are children involved, it is important that they are involved and considered and that the day is not solely about drinking alcohol and having grown up conversations.
But it is not JUST for the kids either. Everyone should enjoy it. Kids don't get to rule the roost and dictate everything. There should be proper food (it doesn't have to be a traditional roast turkey but celebration food to eat together). Adults can sit and enjoy adult conversation at least some of the time. There should be presents for everyone - not an overwhelming pile for DCs and nothing for adults. There should be a mix of entertainment to suit all - so sometimes there will be traditional carols because DGM loves those, sometimes cheesy Christmas hits of the past for DCs to dance to, and sometimes teens can have some of their music just not at deafening volumes.
People at work should each have a fair chance to get time off - not just parents of young DCs but those with older parents, adult DCs, no extended family - if they want time off. And those with young DCs should not guilt trip those who get leave days to change them at short notice.
Parents can take a night out for themselves leaving DCs with babysitters. Not just the work nights they have to go to.
Christmas is a midwinter festival at a time when people generally need some downtime and renewal, quite apart from the religious celebration.
So no, Christmas is not "just for the kids" in any way shape or form. And I say that as an adult child and sibling, a parent, an aunt, a godparent, a working mum, a manager and a person in my own right. Even when dc was small, it was never "just" for them - some years, our family arrangements had to be juggled around their (and others') needs, but as part of that bigger picture.