Old Enough to Volunteer
Wings of Change: Stories About Dragons
Mikal and Tern are not old enough to fight in the war with their fathers, or help hunt a recently sighted dragon. But they are young enough and small enough to help the local mage steal powerful stones from the dragon. But the stones are eggs and the dragon convinces them that the mage has nothing but self-interest in mind leading them to switch allegiances, with the dragon's promise to help end the war.