So, the Cantii or Cantiacii were the pre-Roman tribe of the south-eastern peninsula of Britain circa 55 BCE. The Romano-cantii would have been a part of the much more unified Romano-British: the peoples of Britain after Rome pulled out of the isle in the early fifth century.

As for narratives Nennius (a monk and historian of the ninth century) claims that the region of Kent (Ceint/Kentland) was gifted upon the Saxons by the high king of Britain in return for a marriage with a Saxon leader's wife. From there the story told by Nennius is one off ebbs and flows with the Anglo-Saxons slowly taking over the south and east before being checked by the son of a Roman consul (Ambrosius, Embresguletic AKA Arthur (Gildas writing three centuries earlier says that this man was a consul himself)) at a huge battle at Mount Badden. It was for the longest time believed that this was the fact of Anglo-Saxon "conquest" but u/BRIStoneman has some very interesting things to say about the idea that contrary to this the evidence points more to an elite transfer than an outright population change here: https://www.reddit.com/r/AskHistorians/comments/6q2kv0/cultural_continuance_in_britian_specifically/

Sorry, I've no information about the Kingdom of Kent