These kids are used to having both parents, and many times will want things to go back to the way they were, even if things were bad.
Some parents feel pressured to stay together for the child's sake, However, a family is not really complete if there is discord.
These kids themselves may feel pressured to choose "choose sides" which may cause guilt, and add a burden to an already difficult situation.
Most courts will honestly consider the child's prefrence of which parent they would rather live with, but will ultimatly base their decision on which parent is realistically in a better position the give the child the better life - in their opinion. Ideally, children should have the opportunity to continue to spend time with both parents after a divorce. (Unless there was child abuse or domestic violence.) Remember that whichever parent winds up with custody, will become a single parent household, and will experince the challenges of single parent families. (See: Single Parents.)
"How do I tell my kid that we are getting divorced?"
Don't spring it on them. The more time they have to process it, the better.
Make sure to have the talk when everyone's emotions are stable.
Let them know that the decision to separate had NOTHING to do with them or their behavior.
Let them know that all relationships grow and change, and we can't always control what direction they take.
Let them know that you are taking serious steps to insure their well-being.
Summary
Of course age matters. Older children who are closer to independance will generally accept it easier than younger children. And every kid is differnt, some just handle it better than others. But many children will likely experince some level of anger, fear, shock, and sadness. Although there is no way completely around it, we can do things to minimize these feelings.