My understanding is that teaching will be offered online-only for the first week of term in January, and only for groups of students where this is logistically possible. If your DS/DD needs to be physically back for the start of term's teaching, they'll be able to go back.
I also don't think students will be barred from accommodation at any point - that's madness. Many won't ever have left anyway! Not everyone is going home for Christmas.
Just like the travel window starting today, it's optional. Students can go home now if they wish. But some had already left, and some won't go until after the window closes. All fine. Universities are not physically evacuating students now, and they won't prevent their return in January! Just offering more options to try and reduce the spread of the virus, by letting students leave earlier or return later IF THEY WISH.
No way in hell will students be told to stay at home for several weeks longer, honestly if your kids are told they can choose to go back a bit later it'll only be a week or so.