I will suggest something like, "It's been decided..."
If she's the one telling them, it may go smoother to take a step back and let her figure out and say her own words. You can always add your own if you feel she missed something important.
I have almost no recollection of "the moment" I told my kids I was D'ing their mom. It was a less important moment for sure than our many follow-up talks as things got "real" over the following weeks. I did read a thin book on the subject of explaining the news to the kids that I read and referred to often.
