I suspect that the Middleton's have accepted that christmas with kate without all the fanfare is unlikely and they compromise accordingly. Perhaps they have a new tradition of a huge family event at new year or a pre Christmas event in early December.
What Charles will demand once king is unclear. The queen, for all her more distant parenting style, is nonetheless very focused on her family including children and grandchildren and the cousins etc she grew up with. Charles appears to be less so - v involved and close to his immediate family but less interest in extended family. I think it's probably also understandable when the matriarch has been around for so long- everyone else just focuses on their own bits of it. I mean, we are always been told about the queens active relationship with child x or grandchild y but we dont hear much that suggests that Charles is a doting uncle to A or whatever. And in the nicest possible way, the extended family are getting further and further from him. A child who is the queens great grandchild is only his great niece or whatever.