A Tale of Two Dragons — Geraldine McCaughrean

The kingdoms of Arbor, with its lush forests, and Pomosa, rich with farmland, have no love between them, and the children have taken to sneaking across the borders at night to steal the precious resources of milk and wood that the adults won't share.

So the Kings decide to fight, instead . . . with dragons. But do the dragons have another idea? 

This new parable shows that the way to peace and prosperity is not through fighting, but teamwork and generosity.