In this day and age, divorce is not uncommon. Children hear from their friends about it and why it happens. Expect questions and give them answers. Let them know that they can talk to either of you at any time with any concerns they may have. They will want to know who they will be living with, when they will get to see the other parent and probably a dozen more questions. Before you talk to the kids, talk things out with the prospective ex and work out the answers to these type questions. Also expect the kid that will try to play both sides against each other. They have probably heard about this from their friends and think that they can get away with it. Be strong and stand together, even though you are divorcing.
Try not to argue in front of the kids while working out the details. Yelling and screaming at each other frightens the kids, makes them aware of the situation more and tends to lead them to believe it's their fault. Be adults, show your kids that despite the fact that you can't live together, that you will be there for them, always.