There are no words that anyone can say to convey our deepest sympathy. It is much sadder to see a child pass before their time than an adult. As adults we have had our opportunity to live and experience life good or bad.
I lost a brother when he was 9 and I was 11. It is never something you ever get over from while others move on with their own lives.
My heart felt emotions go out to you and the family. There are no other words appropriate to say, but I am so sorry for your loss.