As others have said, I'm sorry this is where things have got to for you both.
I'm lucky enough to have a twice divorced mother. I was under 1 when she left my dad and in my early twenties when she and my step-dad decided to separate. My younger (1/2) sisters were mid-teens. I firmly believe that parents that don't get on are better parenting apart. Time with both /all my parents is better than it was when they were together, even when they're now in the same room.
My mum and step-dad made the decision to tell us around about now pre-Xmas and then to have a great family Xmas together. It wasn't really a surprise. They intended it to be about how we spent the time together as a family, even though they'd decided not to be together. It set the tone for how things were going to be from then on, and as there was focus, was actually a really great time. They carried on having joint Xmas with us for a couple of years until they both met other people and things moved on. It worked, and the relief of spending time with them when they had a load off their minds was great. It's unlikely your kids haven't noticed what's going on at home.
My wife's parents separated in September when she was around 22. They occasionally get together in bigger family Xmas and for other things. They just seem to exist in each other's space rather than making much effort; it's amicable but void. My wife hates Christmas because she has to make choices about which parent to be with (and still wants the perfect family Christmas that's never going to happen) and it's nothing to do with when she was told they were separating but more to do with the fact they didn't make any effort about how they would be with each other when they separated, perhaps because my wife and sister-in-law were older.
This year might be difficult, next year may also, and in time you'll all set the pattern for how things get on.
My kids have 9 people they refer to as some form of grandparent. It's complicated, but they know they're loved by all. That's the bit to focus on.
Your kids might find Christmas uncomfortable in the future, but it's more likely that it's because it will cause them to have to make choices they're powerless to do anything about and they might look at what friends are doing and wish it was what they were going to have. They're not, but ability for you both to be comfortable in each other's space has to be paramount. You're about to have 1-2 weeks to show them how it could be and it will set the tone for the future. I'd also recommend an alcohol-free Christmas to ensure things are less likely to be said that might be regretted later.
Bottom line, don't use Xmas as an excuse, be honest and show them how it's going to be from now on and how you'll both be there for your kids, together and apart and make it about them not you.