And why would a God use evolution and create defects and fatal diseases in children?
As a means to spend karma.
As a way for the progressing soul to learn certain lessons in life.
As a way for souls to help other learn the lessons of compassion. As the Baal Shem Tov put it: "A soul may descend to earth and live seventy or eighty years for the sole purpose of doing a favor for another. A spiritual favor, or even a material favor." (And that favor might involve having an apparent defect so someone else could learn a lesson in love).