I'm reading a great book right now..."An Exact Replica of a Figment of my Imagination". It's non-fiction, written by a woman about her experience delivering a stillborn baby boy. She writes about her and her husband, but also about how people reacted, etc.
From reading this book and from my own experience with grief they will still be numb. When they said they are still attending it was probably on auto-pilot. On one hand the party will be a great diversion for them on the other hand it will be hard because no one will want to say the wrong thing, etc.
The one thing that has really struck home for me in the book is that the writer WANTED people to talk to her and she wanted their sympathy. She wanted acknowledgement that what happened is awful and sad and that baby was a PERSON to be remembered and loved. Tell the other guests not to tip toe around them, but to acknowledge the sadness, hug them a lot and do things to lift their spirits like telling old stories, etc. All of that can happen in the first 30 minutes and then move on as I'm sure they don't want to belabor it or have it be a pitty party either.
Then afterwards-follow up with them. I bet if you say you'll stay in touch and DO-even with silly emails, will mean the world to them.