I've read two chapters in two seperate books about telling the children. My wife and I plan on telling our 4 girls (9,7,5, &2) next week. Basically the books say to tell the children 1 to 2 weeks before you plan to move out.
As for what you say, basically the books tell you to 1) Not to blame the other spouse;2) assure the children that is not their fault;3) reassure children that you will be involved in their lives just like you are now; 4) inform the children about where you will live, if you know, and when you will see them; and, 5) offer to answer any questions they have. If both parents seem confident , composed and cooperative, the kids will feel less upset, angry, and scared.
My wife and I have set aside a few hours to discuss and rehearse what we will say to the children. Good luck.