Snowtime! (2015)

To amuse themselves during the winter school break, the kids in a small village decide to have a massive snowball fight. Luke and Sophie, both 11 years old, become the leaders of the opposing sides. Sophie and her cohort defend an elaborate snow fort against the assault of Luke’s horde. Whichever side occupies the fort at the end of the winter break, wins. But what starts out as pure youthful fun and enthusiasm deteriorates into a more serious conflict. Joy is restored when all the children decide to attack the fort rather than each other and happily destroy every last bit of the snow fort.

Source: TMDb