Wow, what a question.   As Charles R. Swindol says in his book, For Those Who HURT, "It is the question that hits first and lingers longest."  I honestly cannot tell you specifically why your child died.  I'm not going to pretend to have an answer to all the questions about life.  We could spend all day coming up with the whys but never coming up with the answers.  You're asking why is quite normal though.  We all want to know.  We all want there to be some point to this terrible suffering.   We want to know that our child's death had a purpose.  I can't tell you what that purpose was, only God can answer your "whys".

"When I tried to understand all this, it was oppressive to me till I entered the sanctuary of God; then I understood their final destiny."  Psalm 73:16-17

In her book, In This Very Hour: Loss of a Loved One, Robin Prince Monroe states, "Trying to understand why is a natural part of the grieving process, but there comes a time when you need to let go.  Trying to understand can paralyze you from moving on."  Let me clarify that in no way is she suggesting that you "let go" of the memory of your child.  She is speaking of letting go of that nagging question, "why".  In essence, God is in total control of everything.  Asking Him "why" is alright, but you need to reach a point that you accept His will.