"Never use a long word where a short one will do." - George Orwell
Writing your book is not the time to flex your vocab. Remember when your teacher said K.I.S.S. (keep it simple silly)? Well, don't go getting fancy now. Using "big" words is not only unnecessary, but readers think it's pretentious (see what I did there? LOL!) and often times awkward to read. This tip holds extra true if your audience are children.