I finished the quest by finding all 13 snow statues and catching delta misdreavious, but for some reason when I talk to the little girl (now making the greninja statue with her brother and delta mismagius) she only repeats the same dialogue and doesn’t give me the reward of 100 snowballs. 
Welcome, that is not how the quest works,
Several quests involve collecting large numbers of Snowballs in exchange for items. Hidden Snowballs are scattered throughout several different snowy locations. Additionally, wild Vanillite, Vanillish, and Vanilluxe have a 5% chance to hold a Snowball.
So I have to finish the 55 snowballs quest then get the 100 from the little girl? WTH insurgence 