I remember reading this in middle school and enjoyed the light yet heavy story of this series. Gregor has to make a fly of grown up decisions while still being a young boy. The formula is old and annoying but for this age group I think it works. Boy gets task, boy goes on mission, boy realizes it is not straight forward, boy completes mission and is a hero.
But again this is important to this age because it helps hammer a general idea of how stories work. They understand the when rising action, climax, plot thing even if they don't know those exact words for it.

Great book heavy material handled in an age appropriate and almost light hearted manner. My 5 year old really enjoyed reading this series so far and is excited to read the next one.