My youngest sister asked me to read this book for her (to see if she could). It only took me about a half hour to read through, and there wasn't really anything harmless in it (unless you count snooping on neighbors). It was just a group of kids who were curious about new activity across the street. As an adult reading this, I knew what was likely happening from chapter one, but I can see where it would be enjoyable by younger children.
This is the second Eagle-eye Ernie book by Susan Pearson and Gioia Fiammenghi that we've read and we really enjoy this series. The mysteries are fun and suspenseful, but not scary. And our girls loved that they understood the real story before the characters in the book knew.