Halfcups, if your DC are over-excited, have they gotten enough sleep the night before? One way to try and help that is a Christmas Eve hamper. New pjs, bath bubbles (I get a Santa bomb from Lush for DD), and perhaps a dvd or nice small treat (special Christmas chocolate or cookie maybe). These all come out late on Christmas Eve. If having a dvd, maybe straight after tea, so that it can be watched quietly before bed (or keep for Christmas Day sometime). We tend to have tea, put out stockings and have DD light the Christmas candle, and then produce the hamper so that DD goes off to have her bath and bed happily and on time. And we always read "Twas the night before Christmas" to help settle down once in bed. (I have had longer stories to follow too, another chapter(s) in Harry Potter maybe, which I can read til she's asleep if particularly hyper).
And this far out, you have time to think about what you would like Christmas to be about for the DCs, especially from their perspective. Do they have a chance to play with their new toys? do they have to behave "nicely" for hours before opening presents? do they like Turkey, or even roast, dinners? do they get a chance to have proper breakfast and decent snacks/drinks during day or are they starving by "dinner" time? are they allowed to watch some of the lovely tv that's on (not all, but a few things)?
The treasure hunt idea is good to burn off energy, or a walk somewhere can be magical (especially if there is anywhere where birds or animals might appear). Would they play a board game or some old-fashioned parlour games in the after noon/evening (charades, card games, ...), or work together on a jigsaw etc?
And maybe ask them what they would like to do. So while they might not get all their own way, it's not all imposed on them and they feel it's their day too.
(Is it your gran or STBXHs? And does she have to come to yours, does she stay or can you limit her visit in some way?)
It could be a very good year to re-evaluate the day, and TELL others who don't live in the house that this is what you will be doing in your house (or somewhere else). IF they want to come, your plan is ABC, you are eating X at Y time, and everyone will need to be gone by kids bedtimes. (And maybe have a second story for that night, and possibly another relaxing bathtime, to help settle them down after all the excitements again).
Sorry, that wasn't meant to be an essay or a lecture. (I've had to work hard on some things over the years to change other's expectations).