I'll join in too!
We separated a week ago. H has been abusive, unpredictable, in denial, and is now on a charm offensive. It's not going to work this time, of that I am sure. I told DC last week and I'm damn sure i'm never going through that again. I'm hopeful that he can grow up and behave better for the sake of DC.
For what it's worth OP, I told DC that Mummy and Daddy weren't going to be living together any more, but that they would see lots of both of us, and that we still loved them more than anything in the world, and always would do. I told them that it wasn't their fault, and that there are all kinds of families.
It's super shit.
I think the advice is to be truthful in an age appropriate way. So not too much detail, but enough to satisfy their questions.
H has moved out - staying together over Christmas wouldn't have worked for us (for him, I could have done it). So I'm trying to work out how to have a lovely Christmas for DC. Also i'm going to order a lovely big box of wine to drink guilt-free (H was a drinker) and buy all our favourite foods for us to enjoy.
It will be fine actually. And lovely not to be walking on those egg shells.