Don't Coach: Do not tell the children what to say to the evaluator. Children are usually honest to a fault. Sure, your son or daughter will probably say exatly what you told them to say...but then they will probably follow that up by telling the evaluator, "Mom/Dad told me to tell you ____________________."
Do: Explain the process to your child in a way that he or she can easily understand. Explain that the evaluator is going to ask them questions about what they like to do at each parent's house, school, what their favorite activity is, things like that. You don't have to be very specific because you really have no idea what the exact questions will be. Just make sure that your child knows who the evaluator is and that they'll be asked questions and should answer as truthfully as they can.
Do: Tell your child to talk about how he or she feels and what's important to them, and to answer the evaluator's questions honestly.
Do: Listen to you child(ren), be open with them. Answer any questions as they come up, but let your child(ren) set the tone for the amount of information and detail that you give them. Some children are going to have a hundred questions and will want to know everything about what's going on. Others won't ask a single question. It's all going to depend on the particular child's personality. Also, remember that many details are not going to be appropriate for a child to know about, so while you want to be open and explain as much as possible, you also need to be a responsible gatekeeper of age-inappropriate and/or case-related information.