Full-blown military science fiction in space written for middle grade (9-12ish years old) readers is currently so thin on the ground that The Planet Thieves, by Dan Krokos (Starscape, May 2013) is the only example from the past decade I can think of. Maybe, what with the Ender's Game movie, there will be more...but it's good to at least have this one.
Here's the gist of the story: A 13-year-old Earth kid, Mason Stark, is one of seventeen cadets on board the SS Egypt. They're peacefully logging their required time in space for the Academy, when their ship is attacked by the alien Tremist, enemies of Earth for the past sixty years.
It is a vicious attack, and the crew of the Egypt can't withstand it. Violence and mayhem abound, and soon the cadets (shunted off to the sidelines) find themselves the only able-bodied crew left aboard the ship. And then they discover the reason for the attack -- the Egypt was carrying a powerful weapon that could have won the war for Earth.
Mason, appointed acting captain by the one injured officer still on board, must find a way to rally his fellow cadets to take back the ship, warn Earth, and try, desperately, to stop the Tremists from using Earth's own weapon against them.
But things are not exactly what they seem...and just when Mason things there's might be hope for himself, his friends, and his planet, the dice are rolled again...
So yeah, cool technology, mayhem, cunning plotting, interesting aliens, some neat character interactions....a great read for kids who are only just discovering the joy of sci fi geekdom, and fun for us grown-ups who already know we enjoy it (though it won't break any new ground for the adult sci fi reader). It's also a fine example of the "kids coping in a desperate situation without adults" genre, which I almost always enjoy.
That being said, military sci fi is not really quite my Thing. I skimmed much of the fighting, so I can't offer an opinion as to its quality (I always skim the fighting). I could have used more down time, with more plotting and intrigue, and less zapping, and as I read on, with things snowballing further and further out of control, I felt a tad dizzy. But for those who like heart-pounding adrenaline and desperate struggles, go for it!
(Minor thing that threw me terribly every time it came up--the Tremists have a class of super warrior/wizardly dudes, called Rhadgasts. I read it as Radagasts every time (as in Lord of the Rings)....It's hard (and not fair to the book) when the bad guys become ditsy dudes with brown robes and hedgehogs.)
Note on age of reader: The violence is not sugar coated, the casualty list is long, the aliens are scary, and the stakes are high. I'd give this to a kid before I'd give them The Hunger Games, but it's not going to be every young reader's cup of tea. Fortunately (though not for the characters) the fighting starts quite soon, so it can be put down by those who are off-put by the violence.
Here's a sample of the book, along with a discussion of how its cover came to be, at Tor.