This book was enjoyable but I had a lot of issues with it. It is definitely a book aimed at younger readers with a quite simplistic writing style. 

The concept of the 'twisted tale' (Elsa and Anna grew up not knowing they were sisters) is an interesting one, but the author didn't seem to have really decided whether the book was a true alternate universe to Frozen, or just a slightly altered novelisation of it. There were far too many elements recycled from the film, from lines of dialogue to entire scenes. The use of song lyrics as characters' thoughts was a fun in-joke the first time I noticed it, but quickly became annoying. There was also one particular scene fairly early on in the book which tried to be emotional but I just found to be poorly written and unconvincing. 

The strengths of the book were broadly strengths of the movie too - loveable characters, a true bond between Anna and Elsa, and a sense of magic. 

On the whole I'd say if you're an adult picking this book up for a 7 year old who loves the movie, it's not a bad choice and they will most likely enjoy it. On the other hand, if you want something Frozen-related to read yourself, there is probably better fanfiction available free online.