I was married for just over five years, I got tired of it and just knew that I didn't want to be married any more. I have a son who just turned 5. My divorce was filed in August of last year and was final in November of last year.
Through the entire process we did not argue or raise our voices in front of my son. When I decided to move out, I took my son with me to look at places and included him in all of the decisions that were made that affected him. Before I decided to move out I started talking to him about it. Asking him what he thought if mommy got her own house. My ex helped a great deal in that we never made it sound like a bad thing. We never / still don't bad mouth eachother in front of our son.
All in all the transition was easy I think largely because we kept my son involved in the entire process always being very polite to eachother. When he would ask me about his dad, I would tell him that sometimes mommy's and daddy's don't live together, sometimes they have their own houses. Told him that I still loved him very much and that his daddy did too. I made sure that when I moved out I moved to a place that was close to his dad's house so that if he needed to see me at any time I could be there within a few minutes.
I also took him shopping to buy him new stuff for his new room at my house, a new bed, new blankets etc just to try to help him feel comfortable and at home.
His dad and I still spend every holiday with him and his birthdays together as a 'family'. He has his own cell phone now so if he ever needs to talk to either one of us, he calls.
Honestly his dad and I are better friends now than we were when we were married. The key is that you as adults and parents of a young child need to overcome your differences and learn to be pleasant with eachother for sake of the child whether you are divorced or not. It's important for children to know that both of their parents are there, or can be there in an instant if they are needed. It's equally important for your child to know that there is no anger involved in the separation. If you need to have discussions with your soon to be ex, ask a friend to watch her. Also, make sure that you both let your daughter know (especially if you see any behavior changes) that you both love her, and that this change is not her fault.