My kids adored picture books with big words. They still do, I think. My almost 19YO son still loves Awful Ogre's Awful Day by Jack Prelutsky. Vocabulary includes besiege, cudgel, agitate, commence, tentative, ominous, cascading, bravura, pirouette, panache, pendulous ... One particularly delightful spread reads as follows:
Awful Ogre Speaks of Stature
When elves and gnomes encounter me,
They often shriek, "Grotesque!"
I bow with magnanimity
and murmur, "Statuesque!"
What child wouldn't be delighted with that?