The child actors never saw the "Chocolate Room" before filming. Their wide-eyed expressions when they first entered were genuine.
Roald Dahl, the author of the original book, hated the movie. He disliked the music, the focus on Wonka over Charlie, and even Gene Wilder’s casting. Willy Wonka & The Chocolate Factory
The movie was actually financed by the Quaker Oats Company . They funded the film specifically to promote their new "Wonka Bar" candy line. The child actors never saw the "Chocolate Room"
Despite being a kids' movie, it’s ranked #74 on Bravo's "Scariest Movie Moments" for the psychedelic boat ride. the author of the original book