The legend of King Arthur and his kingdom is well worth knowing about. It was (perhaps) Britain’s best attempt to imagine and describe what is best and most noble about its culture and history (with the help of the French of course who were also a part of its history). Our shared imagination and our storytelling since takes so much from Arthurian legends that any educated child should know about these.
The stories are also packed full with adventure, monsters and courageous knights, so children (particularly boys) are enthralled by them. Many books have been written about King Arthur and his court and you should choose them to be age appropriate, as these shaped were not written primarily for children. This introductory book by Usborne is a great introduction for primary school children. It covers many of the significant plots and characters; it keeps much of the excitement but it is not overly explicit.
Young boys in particular will love these stories and benefit from them.