Kids need to understand why things happen or will happen to their body or mind, they need to hear if from medical folks, and know they can ask questions at anytime. Sometimes Parents don't understand, or don't tell the truth.
Secondly, kids need to know if they are carriers, what genes and outcomes their offspring might have, so that they can make a well thought out future family planning.
Thirdly, kids need to understand and accept who and what they are. I remember a HSchool girl who had Turner syndrome, the school was prohibited (by parents) from ever discussing this with her, even if she asked a question. My point is, kids are curious, they will find out information. Genetic counseling and testing will explain a lot to a child. Counselors are smart enough to couch the information appropriate to the age, and as age increases, make greater information available.