This is just tragic... doesn't matter who's at fault, the point is, lives were lost. Yes it could have been easily prevented, but that's why it's called an accident seeing as though it shouldn't have happened, but it did. Condolences to the families involved.
As a father of two young kids, I do get protective with them when we're up in the slopes. I always have my son go down first while I'm behind him eyeing other people bombing down runs. When he needs to stop coz he's tired, he immediately sits down even on the middle of the run (kids dont think about getting run over, go figure), but that's when I tell him to move over to the side to prevent other people running over us. As what has been mentioned, no matter how good you are, if you can't see whats immediately in front of you physics would tell you there's no possible way of stopping right away.