I think child loss is more painful than other types of losses for three main reasons.
1) You take for granted that you will outlive your children. It is not normal for parents to have to bury their children. It challenges our belief in how life is supposed to be and leads to thoughts of, if this could happen what else could happen. Although we might not acknowledge this thought consciously, I do think unconsciously their is an uneasiness that dwells within us that gets jumbled together with the pain that we feel.
2) When our child dies, we suffer two losses at once. The first is the physical loss of our child and our sensory self being deprived of the feel of their hugs, the sound of their voice, the smile on their face, etc. These things are hard enough to handle, but our dreams that we had planned to share with our child also died. The vacations, the graduations, the weddings, etc, all died with our child as well.
3) When your child is born, you put so much energy into taking care of and developing a loving bond with your child that when your child dies that love has no place to go. As I have said before, Grief is love with no place to go.