A similar tragedy occurred on the airline I work for a few years ago. Every year during our recurrent training, they take a few incidents that happened in the previous year and dissect them to learn from the experiences - this was one of them. A couple with an 8 week old as well as an older child were in first class on a domestic flight. The mom was nursing the baby in a sling while watching a movie, and listening through noise-canceling headsets. If you aren't familiar with them, those things pretty well eliminate all noise except what's coming through the line. At one point she looked down, and her baby was blue. The crew did CPR and got the baby breathing again, barely, but the baby died that day at the hospital they were taken to after landing.
It's been a few years and I don't remember all of the details...but remember wondering at the time if the sling-headset combination might have been at the root of it. If one is nursing hands-free, unable to hear a struggling baby, and engrossed in a movie all at the same time...perhaps tired after a long travel day/etc...well I can see how even the most tuned in mom could miss that something's not right. One of the flight attendants on that flight recounted the event in an audio recording that was played for the class; I don't think I will ever forget her voice. It was clear that telling the story was difficult for her, and she included some of the personal things that the mother had said, the family's reactions, etc. I did not have kids yet at the time, but it was just harrowing to listen to. So tragic..