It's a pretty powerful image. And, like the best artists and authors, Darcy's work leave room for some interpretation, at times searching for tone more than specific meaning.
This works for me for a couple reasons. Many of us associate this case with our childhood, as that moment in our life when the fairy tale ended, when adult concerns intruded on childhood dreams. And because, like our friends Linus and Charlie Brown, Amy is someone who will never grow up, someone who will always and forever be a child. It's our innocence that mourns her. Her abduction and murder was as incongruous to our view of the world as a gravestone must be to characters in Peanuts. It's something that shouldn't be.
Anyway, that's my thought. Darcy did a good job capturing the tone.