I am going to start of with things to avoid saying because if you read nothing else, at least you will have seen this!
Never say things like, "I'm sorry", "Oh, how awful for you", and other phrases that might convey pity.
Don't tell them it could be worse, they have other children who are normal, it is God's will, etc.
Avoid stereotyping. Things such as, "they're such happy children", "they are so loving", "God has given you a child forever", and other such stereotypical sentences are just not helpful, and not true!