So, children can die of sudden cardiac arrest without having shown symptoms before their death? How?

We were discussing this topic Friday in my health cl. Our professor told us that sudden cardiac arrests in children are more common than what people think. He also told us that unfortunately young victims of sudden cardiac arrest often died. He showed us several articles and reports. I found these online. How is it that they die so quickly? Does all blood supply just stop flowing to the heart? I don't understand how a child who seems to be perfectly healthy die all of a sudden.