While the older DCs may be at their DFs for Christmas itself, I presume they will be home at least some of the holidays, and that you will have evenings (or mornings if you do shifts) or regular days off during that time? So once you get whatever roster/work plan you get, have a look and pick a time when you can ask DH to have the 2 smallies and you take the 2 elders for a quiet coffee together or bowling or something age appropriate just with them. And pick another one of those times for you all to sit down and enjoy a family board games night or movie night, or even another "special" dinner (maybe not a full turkey dinner, unless that's what you normally do on those alternate Christmases, but something to mark the season that is a little special).
I agree about letting the sadness wash through first - it is a shock. So don't fight that. But don't let it fester either - think of things you can do with everyone before you go back to work in the weekends running up to the day, and what things you can do in those times of the day that you won't be working.
It could be good to get stuck into preparations now, if you are off work, so that you can concentrate on enjoying time with all the DCs closer to the time. And you also have time now to prep things for that time - buy decorations to be able to make and ice cookies, cut up strips of paper for younger DCs at least to make paper chains, find mad colours and maybe some gems or bits for the older DDs to do festive nail art together, check out reviews to find good Netflix or Amazon festive movies to watch together etc...
And I find that music really helps me to recover my spirit. I start off with choral and orchestral music, which can be more melancholy and let out any glums, and move through country to pick up a bit of a happier vibe up to the more traditional "Christmas Hits" (Last Christmas, Fairytale of New York, It's Christmas, ....the radio playlists). And sometimes, you do have to fake it til you make it - but usually, like the song says, "the result of this deception, is very strange to tell, but when I fool the people I meet, I fool myself as well".....