I'm with Jean. I think you choose the right word for the story, even if that word is challenging (and even better if it's surprising!) In one of my picture books, Maurice the Unbeastly, I use words that might be considered too difficult for children...gargantuan, abominable, and infiltrate, for example. But these word choices add to the voice and overall feel of the book, and it is one of the things readers and reviewers comment on most often. They like the vibrant vocabulary. So I definitely think it can work, depending on the story.