know about losing a child.
The most important thing you can do for the parents and other family members is simply be there. Say so. "I'm so sorry, but just know I'm here for you." That means more than anything else.
If you know their religious belief system and you are absolutely sure about it, and it coincides with yours, then speaking in terms of those beliefs and the understanding of an afterlife may be comforting. But if you aren't certain, then don't use your beliefs to try and comfort them. Things like, "He is in a better place." or "He is at peace." or "He is no longer suffering." may seem comforting, but they are not and may be the opposite.
Trying to make sense of such overwhelming loss is futile. It isn't what they need to hear. Just let them know you are there for them and are willing to do whatever they need in the moment.
I still remember vividly, 43 years later, sitting in the large surgical waiting area with numerous other families in a large teaching hospital where my 6 week old daughter was undergoing an exploratory surgery to confirm a suspected diagnosis of a condition that, at the time, was effectively a death sentence as few lived beyond 18 months. (Fortunately, the survival rate changed drastically beginning that year.) The surgeon who I worked under came out and told us that she did, in fact, have the condition. We spoke briefly about next steps and then he left to go back to her.
My wife and I were left in this very public place, surrounded by many people, but felt utterly alone. We were sobbing uncontrollably. An old black minister, of what faith I do not know, came up to us, introduced himself, and said, "May I sit with you?" He sat down beside us and didn't say anything. He just let us know we weren't alone. That act of kindness remains overwhelming for me to this day.
I can't offer any better advice or suggestions.