Children death is dramatic.
Infant Immortality is a pretty strong convention, and when it you see it averted, it is always strongly affecting the overall mood. One of the oldest martial laws is to never hurt children, since children are innocent. Thus, when a villain is willing to kill a child, he crossed the line and will never come back. When a disease or a catastrophe takes lives of children, you just know that shit got real. If it's totally not dramatic, then it's a Dead Baby Comedy. See also Would Hurt a Child. Since this is a Death Trope, expect unmarked spoilers ahead.