Kodomo no Kuni

In a town inhabited only by children, a terrifying threat appears: the "otona" (adults), who kidnap them. San, whose older brother Haru serves as the military captain combating these monsters, dreams of following in his brother's footsteps and joining the army. However, he fails the entrance exam... Just as he's plunged into despair, Haru's condition suddenly takes a turn for the worse!

(Source: Shueisha, translated)