The city is besieged by the Evil Overlord. Everyone might as well be a soldier as he plans to Kill 'em All anyway, with children being no exception.
Bob was raised by a Camp Follower in an eighteenth century army. He knew no other life but the army so when he was old enough he became a drummer boy. If he survives he will become an infantryman himself.
Inverted: Only the oldest people in this world's society are allowed to fight as they have lived their lives to the full.
Subverted: Children go to great lengths to try and volunteer but an army captain absolutely refuses them on moral grounds.
Double Subverted: The children then become guerrilla fighters and assist somehow.
The actual "war" is a lot less serious, such as a massive scale...waterballoon war.
Zig Zagged: A captain recruits children to fight, then his conscience gets the better of him and he can't stand to force them to fight - looking as though their role in the story will be over, then they rescue him when he's taken prisoner, then he still can't force them to fight, however one is a Teen Genius who devises a strategy to take the enemy stronghold or another is The Hero whose Magic Sword or ability to pilot a Humongous Mecha is necessary to win the war.
Averted: All soldiers are adults.
Enforced: "We need to expand our market demographic. Stick a couple of kids in the show." "But sir, it's a show about bloody war." "Just do it!"
Lampshaded: "My army corps is a goddamned nursery!"
Children are forcibly recruited into the army.
"I look old enough, so I'll lie about my age and join the army to protect my country!"
Exploited: ''Lets get some photos of dead Child Soldiers. It will be sure to be on the front page."
A less notable subversion wherein the children are not allowed to participate in the combat they seek.
Discussed: "We seem to be looking after more and more kids these days, huh Alice?"
Conversed: "Stories about children who fight in war break my heart."
Deconstructed: The ethical implications of this are explored in great detail.
Reconstructed: The Kid Hero breaks out of his Heroic BSOD and plays a major part in an upcoming battle.
Played For Drama: Bob is a Kid Hero who was put on the front lines with his friends. He and his friends always played war games and are exited for the real deal. Then the battle begins, and Bob is introduced to the hell that is war. Within minutes or even seconds, Bob watched his friends get slaughtered on the battlefield. Bob is also forced to kill people during the fighting, but never enjoys it. The war breaks him, and he comes home a Shell-Shocked Veteran in the shell of a young boy.
Eh, you look old enough, and we do need some more warm bodies, so...you're in! Head back to Child Soldiers, soldier!