OP, I am so sorry for what you are going through. It is so unjust but yes unfortunately very common.
I went through very similar.
In terms of telling the kids it really does help them not to be asked to take sides, however it is also important that they can trust what you say. One thing kids often worry about is that it's there fault and being too mysterious about the cause can make that worse.
To me, that means telling them a version of the truth that does not cast one person as the devil/ the other as the victim, but that also reflects somewhat what has happened.
Eg: in terms of whose decision it is, I would hope that you would not want someone who treats you this badly or really doesn't want to be here, so hopefully you do both agree somewhat that the marriage is ending. That is different To saying it is both of your fault.
My dd was a bit younger, but We agreed something like this:
Daddy has realized that he isn't in love with mummy any more. A marriage really can't be happy if that's true. It becomes unhappy for both people. We have known this for a while and we haven't managed to find a way to be happy. We are sure this is not going to change. This means that we are separating. Sometimes that romantic, being in love feeling changes between adult couples. It is totally different from the love parents have for their children and the love we have for you. We both love you completely and forever and nothing about daddy and I separating changes that.
Hope this is helpful.....