When I was very young and newly married, my husband’s friend who married after we did but had a baby before we did, lost that baby right after his pidyon haben so that had to sit shiva for that child. It was heart wrenching. I didn’t really know this woman. I had gone to the bris and so I had met her twice. Once by another bochur’s wedding and once by her son’s bris. Now because he mom lived in Baltimore and her m-i-l had a house full of her own children to take care of, my new husband asked me to go over there and stay with them during their shiva so they shouldn’t be alone.
It was heart wrenching. This young woman kept saying, my son was a tzadekel and the world was just not ready to receive him so Hashem took him back. There must be some mekor that says that because that is what she clung. And I believed it because there is no other reason for a pure innocent baby to die. So it is either for that reason or to be a kaporah for OUR sins. So if you can tell the parents that their child was a tzadik born before its time because this world is not pure enough and good enough to receive him/her that might give the parents some chizuk.