I think it is probably aimed at 8-10 year old competent readers.
I thought this was a really interesting and quite unusual premise for a girls adventure story. It was easy to read but there is a lot of dialogue which I think sometimes interupts the flow of the story, but this may just be my personal preference. The story does move along quite quickly even with the dialogue and it was only the passages about camp that I found myself skipping through.
The heroine (Ingrid) is a good role model for young girls as she's quick thinking and bright rather than the usual emphasis in girls stories on looks.
Although I would have liked a little less dialogue and a little more description of the surroundings (to create atmosphere) I enjoyed this book and would recommend it,
especially to children slightly younger than me as it's quite a slim book and easy to read in a couple of sessions.