Liberator, hopefully you've been able to catch the "Scientology & the Aftermath" series that Leah Remini and Mike Rinder presented on the A&E network. Fortunately, you can view episodes on
http://www.aetv.com/shows/leah-remini-scientology-and-the-aftermath
I have some additional suggestions; the OLD 1981 movie "Ticket to Heaven" is about a guy who got into the Moonies (the Unification Church of Korean cult leader Reverend Sun Myung Moon). A review is here: http://www.nytimes.com/movie/review?res=9B0CE1DA113BF930A25752C1A967948260 you can get a copy from Amazon https://www.amazon.com/Ticket-Heaven-Nick-Mancuso/dp/B000MTEFTQ.
"Ticket to Heaven" came along at a good time for me. Interestingly, the cult deprogrammer Steven Hassan was once a moonie. He isn't any more, of course.
The Mormons are another group worthy of criticism and analysis. One great DVD is "The Lost Book of Abraham", which you can find here: http://www.mrm.org/product/the-lost-book-of-abraham.
Actually, almost anything that promotes critical thinking skills will be helpful, but a multi-angled approach is best, because the kids will learn that the world is full of opportunistic liars who promise everything in exchange for your mind.
Admittedly, some of these films were made by Christian Evangelicals with a goal of gaining more converts. I am not a Christian Evangelical. I describe myself as a "non-practicing atheist." I'm selling nothing. Literally.
Here's a lisy of documentaries about cults from the Internet Movie Database(IMdB):