Read this book before you tell them:
http://www.amazon.com/The-Truth-About-Children-Divorce/dp...
It's enormously helpful and gives you good scripts to use with different ages. Your concerns about their shock are valid. The book explains how in families where the parents have done a good job of being warm and civilized and keeping up the happy family front, the kids can be in shock and feel much worse after the divorce. Conversely in families where the veneer has cracked and the kids know that there is something going on, the decision to divorce can bring about relief and improved well being.
The author poses that the sweet spot is to let the kids know early on in the process that mom and dad are having some trouble agreeing on things and getting along and that they are trying to work things out and make things better. Then if things do get better, great! And if they don't get better and divorce proceedings begin, then the kids aren't blindsided.
It sounds like you are past that point of early disclosure but there is still lots of good advice in the book about what to say to the kids, what to not say to the kids, how to handle your emotions, what you make public and when, how to establish appropriate boundaries with your soon to be ex, etc.
I'm not through the book yet myself (and we're not quite at a decision point yet, still lots of back and forth) but I have found it to be tremendously enlightening and informative.