Did the same thing around 5th-8th grade (not sure when exactly) and had a similar reaction. Though honestly more disappointed than saddened as I had seen the trailer for the movie, but I never went to see it and I had already read several stories like Where the Red Fern Grows and Old Yeller. As a result, I went into the book expecting the sort of straight forward adventure novel that the trailers had promised but was emotionally well prepared for when the did end up tragedy occurring. Honestly no that I'm older I do feel the sadness of the tragedy, but when I read it I was more frustrated that they never actually went on any Narnia style adventures.