"Fireflies" is not intended for small children by any means. It's suitable for pre- and young teens with some parental guidance--kids need to know the historical context.
Teraithia is a pre-to-young teens book. I didn't care for the book at all, frankly, nor did my daughter. But when we saw the trailer for the film we both remarked that it seemed not at all like we remembered the book. The filmmakers have added lots of fantasy stuff that wasn't really in the book at all.
You're right. The fantasy was not in the book. The book dealt with the kids having a place to go by themselves where they could imagine what they wanted to. It was about them, not the world of their invention. I could barely read the book, I found it so dreadful.