Why do they need to ask "why us and not some bad people"?
I think that is a very strange way of approaching human tragedies..

The three years old child that dies from a "good" couple or from a "bad" couple is still a child that dies, with no fault or whatsoever.. it is a tragedy in both cases.
There is not a reason why a child of that age had to die! there are not moral explanations..

The only thing I would say to a couple that lost a child is to let their sufferance out, to cry and scream as much as they need to do, to get themselves somewhere out in a big field and cry out loudly all their pain..
and then, when they've got not more tears left, to turn this pain in something else.. to transform it in something positive, a foundation, a book, an idea of their own..