If possible, both of you need to come together on how and what you're going to tell them. They need to be told as much truth (why, when, how) as is helpful for them to comprehend what is about to happen. They need to be told the answers to some basic questions all kids have: Did we do anything to make you and daddy do this? Where will I stay and with whom? Will you ever be together again? Why are you making daddy leave?
There are many books that deal with how parents should approach this situation. Ask your reference librarian for her recommendations. I like Chapter 7 (Separation, Divorce and Abandonment: Telling the Kind of Truth Kids Need to Hear) in Ava Siegler's book, What Should I Tell the Kids? It gives a fine overview of all your concerns and includes reparative narratives (stories with a beginning, middle, and optimistic ending) to tell your kids. A family therapist may be useful to consult before you begin this process. It will help you develop a blueprint that addresses your children's and your family's specific concerns.
This can be a very stressful time. Remember that how you handle this will have a lasting impact on your kids and your actions and words should be appropriate. If you're going to keep up the fighting and blaming, please get some professional help for the children's sake. This isn't a war to be won. It's an awful situation to be handled with as much compassion as possible.