What to say?

Since we have been staying at the Ronald Mcdonald house I have learned so many things. I have learned that there are a MILLION things that can happen to a child from conception to birth. Things don't develop, things don't form correctly, things get twisted, things grow together and more. We have had 2 couples now lose a child. One was poor Walker who had gastroschisis, he had 2 surgeries to remove the dead parts of his intestines. He was left a small part of his intestines, but then they started to die as well. The parents were told that if they removed the part that was dying that he would not survive- there would not be enough intestine left for life. So, Walker joined our Heavenly Father. Then we became friends with another couple who had twin boys quite a bit early. The boys J & Z weighed in at a bit over 2lbs each at birth and at first seemed to be doing okay. Well, then Z got to where he couldn't really eat. So, come to find out he also had intestinal issues. He had holes in his intestines. So, he went into surgery and the Surgeon gave the parents about a 10% chance of making it. Unfortunately, baby Z has passed away. They still have baby J with them, but of course are mourning baby Z. I have lost children before. I know what NOT to say to them, but then again not really because my children were stillborn vs giving birth to a live child that you are able to love on, kiss, hold, talk to and that sweet little person looks at you, stretches, cries, grunts and makes noises that you can hear. So, what can I say now that will not hurt them. Walker's parents are doing okay. He was mom's first child and dad's second. I have spoken to Walker's mom on several occasians and they have offered to care for Cody so that I may go and see Cami during the day. I have yet to take her up on the offer, but I may one day. With J&Z's parents what should I say or do? They have one son who is alive and things are looking good with him, but then there is poor baby Z. So, any suggestions would be helpful. There is another couple that gave birth to a baby girl named Ella with trisomy 18. Any advice on what to say to them???? Any and all advice would be helpful. Thanks in advance!