As every other Soul out there, there are Souls which choose to experience life on Earth and to finish it as small children due to some illness. It is all part of the great Journey as Souls, where no life is finished but every life is just a continuous transfer of the consciousness. It is undeniable fact that when a child is born sick or get sick little later is very painful process for both the family and the child itself. But on the other side, this process includes so many aspects of the Soul to experience such as unconditional love, empathy, comfort, patience, surrendering. They are all lessons to be learned from that process. Those choices of life after life where humans are suffering while the Souls are learning their lessons It maybe seems cruel to you, but it is no more cruel than what you as humans are doing while on Earth with partial awareness. The difference is that Souls are choosing those kind of life with a much brighter perspective and with a great sense of awareness in opposite of how humans on Earth are choosing their actions.
Try and see it like this. When a director is developing a scenario for its future movie, there is nothing cruel in choosing the actors for the movie no matter the genre of the movie is. Sometimes it is a comedy, sometimes it is a horror. And you still watch it to the end because you know it is fiction, it is just a movie. You know that the actors will continue to live after the movie, right? Well, the Souls know that they will continue to live after their movie on Earth is finished no matter how long they are in this movie. Try to remember that it is all for the greater good of the Soul, it is all due to her wishes.