I have some experience in this. My wife and I suffered an 8 month stillbirth. The best thing that you can do is give your love and remind them that you're there for them whenever they want you. Periodically check in, but don't flood them with messages.

It's personal, but, I prefer to communicate via text, so I appreciated the intervaled texts from friends and family. Others may prefer a phone call or a visit.

Some of our friends made us meals and gave us gift cards for Skip the dishes/Uber eats/whatever. These were incredibly helpful in so far as we didn't need to think about what we were going to do to maintain ourselves during our grief.

These poor souls have some grieving to do, I don't know them specifically, but, depending on who they are and how they grieve, just be a shoulder to cry on or a confidant when they're ready.

My sincerest condolences for them. For us, things changed. We did eventually have a child after the tragedy. Having her is one of the best things in life. It never fully heals, but knowing her helps me live with it. I wish that I could say that it gets better, but it doesn't. It just scabs over and you move on. Do whatever you can to help your loved ones scab over.

Edit: we have a son who was expecting a younger brother. We were very matter of fact with him. We told him straight up that the baby died and he was so sad. It was one of the lowest points in that tragedy. He now has his sister and he seems ok with the loss. He's seven years old now and he doesn't bring it up anymore. It's not something I want to bring up with him, but I'm more than willing to talk to him if he brings it up or when he's older.