I think it depends on what age group you're writing for in the picture book. For older picture books, which teachers use in schools, you can focus on the hero and what s/he did that made that person a hero. That way the tragic circumstances aren't necessarily the take-away the child focuses on. For the very young, say 2-4 years old, I personally wouldn't feel comfortable with tragedy in a picture book.
But that's just my opinion.