I haven't read the books but the two other projectionists who watched the IMAX screening said that the film was pretty faithful as far as they recalled, although it did go into more detail and explanation than the books and included stuff from more than one book. I think films naturally call for a simplification of storytelling, as you only have 2 hours to tell the tale. However, even though I'm a book reader and a sci-fi buff, I still had to pay attention to follow the plot. It's definitely not "kiddied down", despite the fact that it's Disney.
I have to watch the film again. I'm not sure I paid enough attention to Lynn Collins.