Communicating with an ex isn't always easy, but it is necessary when you share children. Being able to do this in a mature and respectful manner is imperative in child custody situations. This sets a good example for the children, and it can help you come up with solutions to issues that put the child's needs first.
Parents who share children shouldn't ever fall into the habit of using the children as messengers. The children should be free to just be kids without having to worry about the adults are trying to work through.
Children who are forced to relay messages between parents might feel as though they have to choose sides. This is a horrible position to put them in because they need to be able to have a meaningful relationship with both parents.
There is also a chance that messages will be misconstrued if the children relay them. You and your ex communicating directly with each other can prevent this from happening, and it helps to ensure that you both understand exactly what's going on.
Another issue with kids relaying messages is that they might be subjected to negative reactions. While the parent might not mean to project these onto the child, some kids will take them to heart. This can be devastating to them.
It's a good idea to have guidelines for communication put into the parenting plan. This lets both parents know what to expect and what's not acceptable. Be sure to carefully consider each point that's put in the parenting plan as each one can outline a different requirement for situations pertaining to the children.