Is anyone still listening to the so called guidelines? Do they even make any sense? My DC will come home somewhere between when they want to, and when we can pick them up, regardless of any rules. They are not cutting short their time in accommodation for which they have paid if they want or need to stay there. The foreign student in DS's flat will have to spend Christmas there alone as it is, so I'm sure he will appreciate the company for as long as possible.
Since the beginning of the pandemic DH and I have kept ourselves to ourselves, allow no one in our house other than our own children, and make them scrub like surgeons if they have been out anywhere. We are not having any visitors over Christmas, there are none of the traditional village festivities taking place, nothing other than a tree indoors to make it different from any weekend, so if my DC want to go out and meet their friends, taking precautions that have now become second nature, I am not going to stop them just because they might be a group of 7 rather than 6, for example. They are only likely to be meeting outdoors, other than DS whose gf has her family's permission to have him at their house, bearing in mind that he has had covid. We have to find a way to live with this virus until a vaccine has been administered to a significant percentage of the population and that is probably going to take the whole of next year.
We are the most elderly people my DC mix with, and do consider ourselves somewhat vulnerable, DH especially, so we keep a low profile and will allow our DC to make decisions for themselves. Rant over.