The Dog Who Stopped the War(1984)

Overview:

As the Christmas holidays begin, a group of 10 war-obsessed kids have a wave of inspiration: What if they spend the next two weeks engaged in a simulated war, armed only with shields, wooden swords, snowballs, and a fierce sense of competition? With a bounty chest waiting for the winners, the two sides gradually grow in size, and the war grows in importance, taking over all aspects of their lives.