I like Dick Price's Divorce "No No's" -- Don't Drag the Kids into the Divorce so much that I ought to just copy his article here. I refrain from doing this but these points are some I just cannot avoid copying to here:
1. Make a child into a messenger. This can be done a number of ways. A note can be sent through the child. A parent can tell the child in person to tell the other parent something. A parent, in a phone conversation, can ask the child to tell the other parent something. However it is done, there is a good possibility that the child will pick up on each parent's feelings (often anger) toward each other. The words used, the tone of voice and other non-verbal communication cues can be upsetting for a child. The nature of the other parent's response, both verbal and non-verbal, will also affect the child.
2. Let kids "overhear" comments about the other parent. This is a passive way to involve the children and subtly try to win them over to a parent's side. It can be distressful for children.
3. Let kids be present, in person or on the phone, to hear arguments about the kids. Parents can easily set up arguments to occur when the children are around, such as when the children are delivered from one parent to the other. It's hard enough for kids to transition from one household to another without adding more tension from an argument.
6. Inform kids, or let them know, about what they are missing out on because they will be with the other parent. This can be a little subtle. Maybe the parent is just disappointed because the child won't be around to go somewhere or do something with that parent, but it's upsetting to the child and it's unnecessary. It's also a way to try to put the other parent in a bad light because he or she isn't doing something exciting or because he or she won't let the child do something the child wants to do with the other parent.
7. Ask the kids to make choices between parents. This can take place on different scales, from choosing activities to choosing who to talk with to choosing where to live. That is too much responsibility for the child and puts the child in an inherently conflicted position. Those matters should be decided by the parents.