Cosmic

Liam, the main character in Cosmic, is like any other kid except that he looks like an adult. That enables him to become the chaperone for a bunch of kids on a secret space flight.

The book is nearly half over before the kids actually get sent into space, but it's an adventure from the very beginning.

Kid who read this book will have to wonder what it would be like to look like an adult and have to act like one to convince real adults they're one of them.

And the interactions between the kids are realistic.  The author, Frank Cottrell Bouce, obviously knows kids.

And when Liam manages to become the chaperone, that's only the beginning of the adventure. Due to some errors, the kids might never get back to Earth, and the trip is secret so nobody would even search for them.

Cosmic is an exciting book that will be enjoyed by both middle grade boys and girls.