Hi op.
I was in your position a few years ago. I'm not saying we couldn't have done things differently or better but this is how we did it (circumstances were that I had found out the day before that their dad had been having an affair and asked him to leave there and then). Dc were 10 and 11:
I told him when he left that I would not be telling the dc on my own so he had to come round the following day and we would tell them together. We sat them down and told them we needed to talk to them. We told them that we loved them more than anything and always would but that we were not making each other happy anymore and were going to separate. Their uncle and aunt had amicably separated in recent years so we referred to that.
We told them that we were still friends (we weren't at the time, obviously but they didn't need to know that and I did damn well at pretending).
We set out arrangements straight away, that they would see see their dad lots and that he was going to get a home of his own where they would have their own bedrooms.
It was heartbreaking, I won't lie. I had only found out the day before that the love of my life was having an affair and now I had to break the DC's hearts too.
I don't think there's any avoiding that.
We answered any questions and all cried and gave them hugs.
Gradually, it got easier and we did become friends and didn't have to pretend any more. Six years down the line we have managed to co-parent really well, always putting the dc first (me more than him usually but he is the one who had an affair so didn't expect much more).
They still don't know he had an affair as I didn't think they needed to know that but if they ever ask the reasons for us separating, I won't lie to them. They had enough information that they needed and could handle at the time.
Good luck op. Put on a united front if you can and plenty of reassurance that this is the best thing for everyone.