I would like to begin by just making an observation that several features of this Gospel suggest that the paralyzed person was not an adult. Jesus calls him “child”, and of course there is the practical aspect of lowering him from the roof: he must not have weighed so much. I do not know why, but I think that when I used to hear this story I thought of the paralyzed person as a man, and while nothing is certain from the indications that I point to, it just seems more likely that it is a child.
If it is a child, it is all the more difficult to deal with the association that Jesus makes between the forgiveness of sins and healing. A tiny paralyzed child small enough that four people can let him down through the roof: is he being punished for his sins? What sins could this child have? The obvious answer is original sin.
Perhaps this seems unfair or cruel, but that is really a separate question. We know from reality that suffering is present from the first moments of life. A child born with a disability or disease, are they guilty of some sin? Of course not! But then again they are often suffering the consequences of sin. Fetal alcohol syndrome, addiction to drugs, malnutrition, and many other ways that young children suffer is the consequence of sin — their parents' sin or the sins of their country's leaders or the sin of greed and selfishness in the world. Since all this is apparent on the material level how can we doubt that it reflects a spiritual reality that sin causes suffering in this world, and the innocent are often the ones who suffer.
Yet Jesus says to the child, “Your sins are forgiven.” Original sin needs to be forgiven just like actual sin. In what way each of us carried the guilt of original sin from our conception is not clear, but we know that baptism does forgive the guilt of original sin. Undoubtedly, there is much we do not understand about reality. There are more things in heaven and earth, than are dreamt of in our philosophy. When we see a child suffering we can be sure that that is the consequence of sin in this sense that no child would have suffered in Eden. All the suffering in the world, what we hear on the news, what we see with our own eyes, is the consequence of sin, of our sins, and only Jesus has the power to save us from our sins.