While breaking the news of a divorce isn't an easy task, honesty and calmness are important when discussing it. It will serve no use to speak poorly of the other parent or speak poorly of the marriage in general. It can be as simple as telling a child that mom and dad are deciding to get a divorce because they can no longer live with each other.
It is important that a message of this sort is followed up and emphasized with the reassurance that both parents love their child very much and that part will never change. Assure your child the divorce has nothing to do with them, because ultimately it doesn't.
Children are smart and intuitive, and they will most likely suspect trouble far sooner than you would expect. That is why honesty is the best policy. When their fears of divorce do become confirmed, there needs to be an open line of communication. Divorce is the dissolution of a marriage, but a positive parenting structure should always remain intact.
Source: Huffington Post, "So You're Getting Divorced. How Do You Break The News To The Kids?," David Wygant, Sept. 7, 2011