Torah Guidance: How can it be, a child that hasn’t sinned, suffering from a fatal illness (God forbid); a child snatched from the world?
A possible understanding is because he needed to correct something specific from a previous incarnation, and when he completed that (‘tikun’), he returns to his dwelling in the upper world. These souls (‘neshamos’) that come back to this world did very few sins in a prior incarnation. Those that are completely wicked are judged in “Kaf haKela” (a place designated to someone not merited entering into Hell, and this is the greatest punishment). Therefore, the person needs to be exceedingly cautious not to sin further.
Children severely stricken physically and/or mentally are exalted souls
that needs to correct something very specific in this world. Now it is known from above, that if they were to be ‘like everyone else’ they are liable to sin further. It is because of this that their steps are limited. Unfortunately, not after a long time, they return to their lofty place.
The Chazon Ish was accustomed to stand when a mentally retarded child would enter, owing to the power of his lofty soul.
A person does not know for sure what he was in a previous incarnation  (except for someone like the Arizal Hakadosh, etc.) This needs explanation. If the purpose of incarnation is to correct something, why doesn’t he know? The answer to this is one of the great principles in belief (faith in God). God runs His world with reward and punishment. If a person could only do good, why should there be a reward… he could not have done bad; alternatively, if he could only do bad, why should he be punished, he could not do good!
He is given free choice. Why? Based upon this, to do good or bad he is judged. If he knew definitely what he was in a previous incarnation (unless he is masochistic, etc.) of course he will make the ‘right’ decision-he is being forced to. God does not want this. God gives the person free choice, based upon his belief, what is the correct action, and there must also be an element to think otherwise as well. God does want the person to choose the right thing to do, the good, because He is the Master of loving kindness, and wants to reward the person. Nevertheless, if a person examines his particular difficulties, his weaknesses, areas which he is more exposed to than others, it is highly probable that these are what he needs to correct.