I am so sorry for this family's loss but was there no supervision?
I have taken my Troop out a number of times where there are, say, 3 leaders and 18 kids. It's easy in such a case for a few kids to get separated from the leaders' direct supervision for a few minutes, and that's all it takes.
We take Scouts up to Devil's Lake in Wisconsin for rock climbing. Once we climb up to the top of the bluffs, and before any activity starts, I usually give a speech along the lines of "This is a dangerous activity. When you are at the park district, all the sharp edges have been filed down and padded, the water fenced off, the glass picked up off the fields, and everything is done so that no matter how stupidly you act, you won't get hurt. Not here. This landscape is beautiful, but it will quite uncaringly kill you if you fool around for even a second. People die here every year, and it's always from doing something against the rules. I've explained the rules. You will follow them, to the letter, and you won't try to game them. Or you will be sent home, immediately, and I won't care how much your parents will complain to me."
It's usually quite effective.