"Fallen Walkers" by Wells Taylor Dyson
Far below the surface of earth, a group of children engage in a secret government program. They are trained to pilot Mech-Walkers, which are giant mechanical insects. These kids protect Earth from hostile space barbarians using their robo-insect powers. Vander, the leader of the Mech-Walkers, mysteriously disappears in combat. Now the Mech-Walkers need to find a new leader. Seth, a boy from the surface, stumbles into Mech-Walkers while being chased by bullies. Now he has become an unlikely candidate.