Talking with your children about your divorce is a difficult but necessary evil many of us go through. It is important you have an open and honest dialogue with your children, but remember they are children. Keep your discussion on their level. You can be honest without disparaging your soon to be ex. Here are some good tips I found on HELPGUIDE.org: http://www.helpguide.org/mental/children_divorce.htm
Tell the truth.
Your kids are entitled to know why you are getting a divorce, but long-winded reasons may only confuse them. Pick something simple and honest, like “We can’t get along anymore.” You may need to remind your children that while sometimes parents and kids don't always get along, parents and kids don't stop loving each other or get divorced from each other.
Say “I love you.” However simple it may sound,letting your children know that your love for them hasn’t changed is a powerful message. Tell them you’ll still be caring for them in every way, from fixing their breakfast to helping with homework.
Address changes. Preempt your kids’ questions about changes in their lives by acknowledging that some things will be different now, and other things won’t. Let them know that together you can deal with each detail as you go.
It’s vital to be honest with your kids, but without being critical of your spouse. This can be especially difficult when there have been hurtful events, such as infidelity, but with a little diplomacy, you can avoid playing the blame game.
Present a united front. As much as you can, try to agree in advance on an explanation for your separation or divorce—and stick to it.
Plan your conversations. Make plans to talk with your children before any changes in the living arrangements occur. And plan to talk when your spouse is present, if possible.
Show restraint. Be respectful of your spouse when giving the reasons for the separation.