Just stick to the facts. If it doesn't directly affect the child's own living situation, I would just leave it at Auntie Mary/Grandpa Joe/Cousin Rick/whatever is going to be getting a new house and living there now. When it comes to events or activities that they won't be together, again just stick to the facts-"Auntie Mary will be with us but Uncle John won't be coming." At this age, they will adapt to the change easily.