Actually, the words aren't so much big as underused. On that basis, they can be unfamiliar. I'm not a fan of using big words when little ones will do. However, as a culture, we're rather lazy with language. Lack of variety and imprecision abound.
Books are the last repository of specific language.
So, when a more exotic word refines a phrase or adds subtlety and grace to a sentence, I'll give it the nod over its more common cousin. The second ingredient is mission. The execution of this mission forces the characters into a conflict, which, to solve, demands they stand, if not on their own resources, on principles beyond themselves. Then, if I've adequately humanized the characters in the few broad strokes allowed within the text limits of a picture book, the readers will not only willingly suspend disbelief, but give the story their heart. They will invest themselves emotionally.