Your'e correct that the trope isn't simply "Children's books where somebody dies." But you're not correct that the defining feature is "it's about such books that win awards, are widely assigned in schools, are praised and touted by adults."
The trope is plot and characterization trope. I've bolded the important parts of this (extended) definition:
At the end of the story, this very special best friend is abruptly killed off, usually in a clear-cut case of Diabolus ex Machina. A favorite trick is to have the death happen entirely off-screen. The more horribly poignant the tragedy the better.
All this is generally accompanied by lots of "end of the innocence" angsting from the main character, along the lines of "That was the day my childhood ended..." Really, it's just the author's way of having a child suddenly make the jump to adulthood via a single defining tragedy. "
Whether a book has won an award or not is irrelevant to whether it's an example or not.
edited 22nd May '14 5:07:51 PM by Madrugada
Bold is not always easy to spot in a quoteblock. Maybe we can pare the summary down to:
Ok, But it's not "usually" as a metaphor for coming of age and loss of innocence. That's an integral part of the definition. So is the fact that it's a Slice of Life story. It may be a side story on another type of work, but the hero's best friend happening to die in something like a war story or heroic fantasy story isn't Death by Newbery Medal
edited 22nd May '14 7:05:08 PM by Madrugada

I would like to go through this page and remove nonexamples — because the trope isn't just about children's books where somebody dies, it's about such books that win awards, are widely assigned in schools, are praised and touted by adults.