My advice for explaining a custody evaluation to the children is something like:
You know that mom and dad have had trouble agreeing on some things since the separation, and we have asked for a custody evaluator to make some recommendations about what would be best for you. He will be meeting with mom and dad, and he will come to dinner at both homes while you are there, to see where you live and how each household operates. He will also meet with you in his office to discuss your experiences and have you fill out some questionnaires that are designed for children your age. When you meet with him, feel free to say what you truly believe, and answer his questions honestly.
The important part is to not involve the children in parental conflict, and not share information that puts forth your “position” or makes the process sound “scary.”