I know for some families, separating the children from one parent or another is a necessity - physical abuse, sexual abuse, perhaps the parent is a drug addict, etc.  
 
BUT in so many cases I see, the problems with custody have nothing to do with such a thing but rather, one spouse wanting power over the other, anger, resentment and revenge.  
 
It is the case with my sister. She and her ex have been fighting over custody since their daughter was 18 months old. They cut each other down in front of their daughter. He calls her mother stupid and lazy, my sister calls my niece's father fat and dumb, and the list goes on. They drag this girl into every fight they have.  
 
As a result, she became ill, trying to please both parents - and all they did was blame one another - "Well, if you weren't so stubborn, she wouldn't be worrying herself sick!" and such. She was ten years old and diagnosed with a BLEEDING ULCER!!!  
 
I finally took custody of her for several months and she began to see a therapist. This woman contacted my niece's parents and after several months of therapy and tears, the two of them finally began to see just how hard it was on their daughter - they did agree to stop dragging her into their petty arguments and trying to pit her against the other, though they still argue.  
 
My niece, on the other hand, has learned to NOT try to please her parents the way she once did. Now, if they try to drag her into an argument or one parent tries to change the scheduling at the last minute, my niece tells them "NO! This is my week with Dad (or Mom). If doing this with you is so important, reschedule it for next week when I'm back."  
 
I am so proud of her and my husband and I stand by her and always will. She's become a confident young teen (13) despite her parents selfishness - and in this situation, that is what it is, selfishness. While they love her as any parent loves a child, in all their anger and hatred towards one another, they see her as another piece of property to argue over. They've forgotten that she is not property - she is a precious little girl full of life and dreams and love for her parents. They've forgotten that at one time, they did love one another and choose to bring a life into the world together. They've both forgotten that they EACH made the choice to become involved with one other and are now forever tied together by this child - whether they like it or not. And maybe that is the bottom line for these two - like it or not, they are forever linked.