Not at all modern, but I enjoyed Rosemary Sutcliff's 'traditional' retelling of the Arthurian legends when I was in middle school (especially the first volume "The Sword and the Circle" -- the others are a little more 'emotionally mature' despite being well within the technical reading ability of a precocious child. Most children don't enjoy tragedy much, and the Grail is bittersweet and the downfall of Arthur's reign downright tragic.) And they are still in print, and still getting rave reviews.
https://www.amazon.co.uk/King-Arthur-Trilogy-Circle-Camlann/product-reviews/0099401649/