What are the tiny little worms killing snowball tree?

It could be larvae of Moths or Leaf Beetles. More specifically, some examples of species feeding on snowball tree leaves inculde: Paleacrita vernata and Pyrrhalta viburni (moths), and Hyphantria cunea (Leaf Beetle). There are other possible species, but these are among the most common. You should eliminate them with proper insecticides as soon as you detect them.