The main characters are obviously Lamia and Merlin. Lamia is often depicted as a unique and mysterious character with her own set of powers and abilities. She might have a connection to the more shadowy or less - known aspects of magic.
Merlin is the central figure in most Merlin kid fanfiction. But other characters also shine. For instance, Morgana as a child can add a different dimension. Her early days and how she might interact with Merlin can be a great plot. Gaius is another important character. In kid - themed fanfiction, he could be like a mentor figure to Merlin, teaching him about magic and the world. Also, the knights in their younger forms can be part of the story, perhaps as friends or foes of young Merlin.
The characters would likely have a range of reactions. Merlin himself might be both flattered and a bit embarrassed by some of the fanfiction. He's a humble character, so seeing all the attention on him in these fictional stories could make him blush. And for the knights, they might laugh and joke about it, while also being a bit curious about how the fans view their adventures.
Obviously, Harry himself is a central character. Then there's often Hermione, who helps Harry figure out the implications of being Merlin's heir. Ron is also there for support. Dumbledore might play a crucial role as the wise mentor guiding Harry through the discovery of his new identity.