My clinical research has focused on developing health education tools and strategies to overcome stigma. On a personal level, I grew up with a brother with Down syndrome, and recently watched my father slowly lose his brilliant mind to Alzheimer's. Perhaps this is why I am particular aware of the complexities involved in telling children about Huntington's in the family.
Truth is better than deceit, and honesty fosters trust and a sense of security.
In other words, never evade your child's questions and never lie when you answer. Before jumping in to answer a question, always take time to clarify what the question was really about. Forewarn the child of anticipated changes in the affected parent's role. The doctors are trying to help him with this sickness, but he won't be strong enough to play ball with you for a while. Write it down.