We’ve just had to do this after my H decided he no longer wanted to be in the marriage 6 weeks ago. I was terrified at the prospect of telling DD5, but the worst bit was the anticipation as it turned out.
Because I was still reeling from his out of the blue announcement, and due to the fact that we had such different perspectives on the split (he claims he’d been unhappy for ages; I thought things were pretty good), we got the help of a mediator to facilitate me and H coming up with a sort of script we’d tell our daughter (and also to help us negotiate how 50.50 residency would work in practical terms).
I was insistent that what we said had to be a brief and honest age-appropriate version of events. Whatever we said had to contain enough of the truth to stand up to DD’s questioning, without placing any blame or finger-pointing. By honestly standing by the news, I could show that I was (sort-of) OK and calm with it. I knew she’d look to me guide her emotions, i.e. if I was OK, the more likely she would be too.
We pointed out a few changes that had occurred recently, and then talked about how Daddy has been unhappy. We said that he didn’t want to live with Mummy anymore, so he’s going to live in a different house where he can be happy, somewhere where DD can live too for some of the time. We will still be a family and Mummy and Daddy will still talk to each other, but they won’t live together again. We said that although it’s sad, it’s not necessarily bad.
The lots of reassurance - it’s not your fault, you will always make both of us very happy, we’ll all be OK in the end, you can ask questions any time, etc.
In the event, it went well as it could have done, I think. A bit of everything from DD - surprise, sadness, lots of practical questions about what she could take to Daddy's house, even a bit of excitement at the novelty. She asked to see his flat, so they went there pretty much as soon as we'd finished talking, and to buy a few bits for her bedroom (very exciting). I had to stem a few eyerolls at what he was offering her – anything that made it easier for her.
Remember it’ll be a series of chats and questions. DD didn’t seem to process it all for the first few days then gradually started asking questions.
Good luck (and sorry if this is too late!)