One thing people leave out, is the fact related to reincarnation and karma. I soul comes into manifestation in physical form, and has karma to deal with, or has a life mission to accomplish while in this embodiment. If a child develops some sort of malady, it behooves us to realize these things don't happen in a vacuum, there are many influences surrounding what goes on within us and outside of us which color how the life experience is experienced. So, it may be a soul agrees to come into this manifestation to clear up some karma, maybe related to his/her own baggage, or to effect how someone else proceeds in their life, which is triggered by the child's malady, etc. It isn't a simple thing, there are many threads which make up the cloth of this manifestation, and each one is essential, just like the pieces of a puzzle, to out picture the whole expression of God/Source.
Whatever is upon a person's path in life, is there for a reason, to help us learn more about ourselves and our place in the whole scheme of things. We each have to come to a point where we acknowledge then take control of our journey, take the helm of the ship and steer it to our desired destination, rather than letting the currents of life buffet us and move us where we do not choose to go.