Sorry to hear that Miss C.
On the plus side such children are very loving (tho' one has to be careful as they are so trusting) and those of my acquaintance do not seem aware that they are in anyway "different". One little girl locally has progressed, at her mother's insistence, right through nursery, infants, junior schools and now is at High School mixing it with all the other children. Only the odd one has bullied her at all and they have been stamped on by all the other kids who, like children the world over, have a moral compass of their own in such matters. Katryn (for such is her name) is an affectionate and trusting child who works hard to "keep up" and seems unaware that she cannot and never will. Her parents know that she is probably only going to have a short life but they determinedly make it as normal as possible for her (she has two brothers, one older and one younger) despite the hard work it involves. Katryn's mother, in particular, is merciless at dealing with teachers, social workers etc and ensures that her daughter gets the same treatment as all other children whilst at the same time she seeks out all the help she can in the way of a special needs helper at school etc.
I do recall the school tried to rule Katryn out of the School Sports day and her mother went down and caused a hell of a fuss until they relented. It brought tears to my eyes to watch Katryn laughing all the way round the running track to come in last by a country mile whilst all 200 children stood on the benches to applaud her home.
That family look upon Katryn as one of "God's chosen few" and why not?
What can you say to the parents? Try "Oh dear, well if that is what God has decided then you must make the best life you can for him, it will be hard work but remember I am here for you and the fact he is a Downs child is no reason that he should not be cherished. He is not different, he is special. His time on Earth may not be as long as you would have wished so make it the best time you can"