Arthur can beat Kay as a kid

 Arthur is at a tournament with Kay as his squire and second and Kay has SOMEHOW gone all the way to the fight and left his sword. Being a kind and gracious big brother, he blames Arthur and tells him "Go get my sword from home, or I’ll box your ears!" And Artie, knowing he could beat his brother with both hands tied behind his back, but that would be rude, goes off.

Arthurian Myths Retold pg 22