Where to start... Based on the trailers (and the fact that it's Arthurian-related!), I had been really looking forward to seeing The Kid Who Would Be King last winter, except it came and went in the cinema so fast I missed out on my chance. And now I'm kind of glad I didn't see it because owww... I don't know if it was because I wasn't in the right mind frame when I watched it last night (and half asleep), or I've finally outgrown kids' movies (boo!) or it's just not that good? I will say that the lead kids themselves were pretty good with their roles, and fit well together as duos and then the team. Particularly Louis Ashbourne Serkis, whose Alex is a fitting stand in for a young King Arthur.
For me the reference points from previous Arthurian tales for this movie are clearly T.H. White's The Once and Future King, or for the kids watching but who haven't read it Disney's The Sword in the Stone, and also the film A Kid in King Arthur's Court (for the mix of medieval and modern warfare tactics, but in the present instead of the past). You could imagine it as a centuries-later sequel to the animated film. It even starts out with an animated retelling of the main points of the Arthurian legend - with an added level of fantasy with a stronger and more evil sorceress Morgana who swears to return to claim the land once Arthur is gone and no longer remembered and basically the world has gone to shit (wars, famine, greed etc. in abundance - so basically present day).
Obviously, spoilers below!