In the tradition of european fairy tales, children were not immune from coming to harm. That would classify it as a dark book by today's standards.
Well, yeah - they do sortof come to harm, though we are told, even in the book, that they'll be fine. It wasn't ~scary~ and dark. I haven't seen the trailer, I'm not really interested in seeing this, but the level of ~scary~ in the first movie is all that was ever in the book. It wasn't a suspense thriller or anything.