First of all, I don't envy you who read this book as a kid. 
 Really, if I read this book twelve years ago, I would not know what moral lesson I could extract from the story. Even now, I am not sure. 
 One thing for sure : my heart aches. Even with all the preparation I did when all my friends who knew I was going to read this book said, "Uh oh, you will not like the ending." My heart aches still. 
 'Bridge to Terabithia' remind me of a life as little kid, with secret imaginary world so full of magic, like it was going to be a reality if you will it strong enough. And how that fantasy of escapism could be ended abruptly, and somehow you got to grow up. And in the end, you grow up. 
 But still...a piece of Terabithia lives on inside your heart.