I really don't think parents should explain to kids at any age the reasons for a divorce. Sometimes it will be obvious to them, but other times not. In my case, I left my first husband (the kids' dad) because the marriage didn't feel "right" to me. I still don't entirely understand all of the reasons why, myself--even now. I still feel the same way about the marriage, and I still care very much about the man, but how would one explain that to a child?
In addition to assuring the kids that it isn't their fault, I think it's important for them to know that both parents love them and will continue to take care of them. The thing kids worry most about, I think, is "What does this mean for ME?" Will I have to move, change schools, will I still see mom/dad, can we still do fun things together, where will I be for holidays--that sort of thing. And maybe all the details haven't been ironed out yet, but reassuring them as much as you can that things will stay as "normal" for them as possible is a good thing.