I've been thinking about when my grandmother died. My Nana. She was 92. She didn't cease being a person when she died. She didn't stop being my Nana. No one said to me, "I guess you weren't meant to be a grand-daughter." No one said her life wasn't meant to be. No one even remotely implied that I should try to forget her. My GP didn't say, "It would be best for your daughter if you moved on and had another grandmother." And when she died, no one said they couldn't find her heartbeat. She was simply allowed to have died. And she's been allowed to maintain her place in the family ever since.
At school the other week, one of the other parents asked me if my daughter has any siblings. I said, "Yes she has a baby brother who died." I showed her his photo on my phone. I am Quick Draw McGraw with that thing. She was compassionate, offered her condolences, then after a few breaths asked "How old was he?" Not, "What's his name?" There may be many innocent reasons why she asked this, but my griefbrain translates this as: "How sad do I need to be about this?" Just accept that it is sad. It won't kill you. It didn't kill me (not literally) so you will be fine.
When friends are looking for ways to support bereaved parents, one of the biggest things they can do is to accept. Accept that child as a person. Accept that child as a member of the community who has died and everything that entails. Accept that that child has his or her place in the family and that can never be altered. Accept what the parents are saying about their experience. All of it, not just the nicey-nicey easy to digest I'm-so-transformed-by-my-child's-life stuff. The crappy stuff too. Feel free to extend that to anyone going through anything difficult. It is their experience, accept what they are telling you about it. Accept your own sadness about it.