How would you start a letter to a family telling them their child has a genetic disease?

In person...: I would never convey that kind of sensitive information in a letter...Face to face would be my call; with preparedness to discuss all the ramifications and options to the family of said "disease".

Always Face to Face: I always make the time to give this information personally. I have also found that doing it the format of a house call is the most effective, unless the patient happens to be hospitalized at the time.