Well, of course, there's the little vampire himself. He could have a best friend who is a human, a brave kid who is not afraid of the vampire's nature. There might also be an old vampire mentor who teaches the little one about vampire traditions and how to control his thirst.
One great 'Little Vampire Gregory' fanfiction is 'Gregory's New Adventure'. It takes Gregory into a modern city and shows how he adapts to new technology while still being a vampire. It's really interesting to see how the author blends the traditional vampire traits with modern settings.
One common theme is the struggle between the vampire's nature and the human world. Since they are vampires, they have different needs like drinking blood, but they also want to interact with humans. Another theme is friendship. Often, a little vampire will form a strong bond with a human or another supernatural being. For example, in many fanfictions, the vampire has a human best friend who helps him navigate the human world.
There could also be a character like a wise old vampire in the community who acts as a mentor to Rudolph, guiding him through the difficulties of being a young vampire in a world that has both vampire and human elements.
One interesting storyline could be about the little vampire's adventures in the modern human world. For example, how he tries to fit into a school without revealing his true identity.
One common theme could be the exploration of the relationship between the two male characters. It might involve how they overcome differences, like one being a vampire and the other being human. Another theme could be acceptance, both within themselves and from their families or friends.
One great Vampire Knight little girl fanfiction could be about a young girl who discovers she has a special power in the Vampire Knight universe. It might involve her befriending some of the main characters like Zero or Kaname in a really cute and innocent way.