Although I can imagine cases where comparing the value of lives might be the way to go, it's not obvious that this is one of them. Heading down a path where we value lives by discounting on the basis of the likely number of remaining years (which is all I see at work here) seems a very dubious idea, fraught with all sorts of moral peril. Although there is something particularly poignant about the death of a child, this doesn't simply translate into a case for saying that the best solution to the dilemma you pose is to give the child's life a weight greater than that of the five adults who would otherwise die.

All this said, there are some hard issues in the general neighborhood. Deciding how to use resources in end-of-life situations, for example, is a serious problem where some sort of discounting doesn't simply seem out of place. But the issues here are tricky, and it's hard to see how any simple rule will work.