There are subtle differences between Myth, Legend, Fiction and Historical Fiction children will eventually need to grasp. The legend of King Arthur is a great way to introduce the differences between Legend and History precisely because there is not definitive evidence that the Story of King Arthur is nothing but pure fiction, yet many of us, including historians, would like to believe there is some reality or at least true person or persons that inspired the tales allowing us to call the stories Legends rather than Myths.
So who was King Arthur and Where do the Stories Come From?This fabulous documentary "In Search of Myths and Heroes: Arthur" Goes over the possibilities vs. the realities about how the many myths of King Arthur and his knights arose. The documentary is also available on Netflix Streaming. If you can, I recommend watching it there for better picture and sound quality. It also discusses why the stories are so historically important despite the absence of solid evidence in any truth behind the tale.
Quest for King Arthur as narrated by Patrick Stewart and from the History Channel was an interesting viewing though the very beginning section goes over the lustful intrigue behind Arthur's Birth quite clearly so as I usually recommend, you will want to preview the movie before viewing it with your young child. Though clear, the discussion is not excessive nor are there explicitly clear images so it is almost certainly fine for your average middle school student or older.
Histories Mysteries has also covered The Knights of Camelot. The movie doesn't truly delve into the question in any depth or detail, but it does offer a summary of the highlights of the history of the tales and the re-writes that have taken place over the centuries.
Make sure you actually read at least some of the tales. They have been written and re-written many times over and are available in versions for any age from about second grade on. I have completed a short article about using the Legends in a literature lesson or two that includes a few resource ideas on books to use. Whether you read "Le Morte D'Arthur" by Thomas Mallory, together with your highschooler, or you pick up "The Kitchen Knight" by Margaret Hodges and other Arthurian picture books for the youngest Arthurian pupil, having read some of these tales, will make further reading and movie watching that much more enriching. So many of our modern "quest tales" have been influenced by the Arthurian Tradition, you will start seeing it oozing out of even modern movies.
Writing their Own Legends and Tall Tales:Have your kids take a real-life historical figure about whom we know a lot and turn that person into the basis for a legendary tale or tall tale. Start with a look at an already written legend like Robin Hood (who was probably really an amalgamation of many real-life people) as well as a more modern legend such as Paul Bunyan or Pecos Bill as examples. Compare these legends to The Legends of Arthur and his Knights of the Roundtable. Then, let your kids imaginations run wild as they figure out how people would have exaggerated the characteristics of the well-known historical figure and his/her exploits in history. A take on this assignment that would be great for your teen or teens is to have them write a skit about the famous figure in keeping with the spirit of Saturday Night Live that makes fun of the "Super Powers" that make this figure "Legendary".
Online Resources for Lessons and Activities Related to Arthurian LegendWeb English Teacher has a whole page with links to a variety of lessons, activities and ideas for different ways to go about using Arthurian Legend in your classroom. If you, like myself, are a homeschooling parent, don't be afraid to check it out and make alterations to suit your needs too. Some of these lessons have more to do with Medieval History, than with Arthur, or with Myths and Legends themselves, but many of us like to include cross curricular components to what we do with our kids, so all of the links here may be helpful.
Early British Kingdoms, is a page specifically designed for primary school kids so it is especially good for your youngest "schoolers." It has broken up Arthur's Life story into easy to understand sections for kids to study. You can also purchase related activity sheets on the site. I did not personally purchase any of the sheets, but the samples they show look decent enough. If you do make a purchase, please come back and add a comment about their service and your satisfaction with the quality of the activity sheets.
Lesson Pathways, This site is another that provides links to a variety of activities one might do with a child while learning about Arthurian Legend. Of those linked here, I'd have to say this is my favorite site. The idea of having kids write a resume to "apply" for a position at the round-table is just one of a number of great lessons offered here.
An Archaeological Quest for Arthur: This Final Site is really one with more information about the History and Mysteries surrounding the Legends of King Arthur, what is known and how it is known. A good resource for students learning research skills while studying Arthur.
Britannia: Links to more information on King Arthur