The kids: Each kid is introduced with a caricature of bad manners (except Charlie). Each kid (except Charlie) is also accompanied by a sycophant parental chaperone who wittily or no, encourages that abhorrent trait in their child.
Willy Wonka: For most of the movie Wonka comes off as though he's everything short of murdering children. And for most of the adults watching it doesn't seem like a huge deal, which is kinda strange, especially since this is supposed to be a kid's movie.
Why that works: If the kid's performances hadn't been spot on, Wonka sending each one to certain death would have played as though he was a callous serial killer. But because the bratty, single minded, overindulgent and greedy characters are portrayed so well by the children (excluding Charlie), Willy Wonka comes off as less a killer and more like he's living out our darkest adult fantasies about how we would like to deal with ill-behaved children and their parental counterparts...
That's a shame: When the film came out critics who liked the film also glossed over the amazing performances from the kids, failing to even mention the performers by name.
Based on a kids book: Director Mel Stuart's 10 year old daughter was reading Charlie and the Chocolate Factory when she suggested he make the story into a movie with producer David L. Wolper. Wolper agreed and secured funding from the Quaker Oat's Company and they adapted the book into a musical. Quick recap: a 10 year old girl is responsible for this movie's existence!