One possible theme could be destruction. Kaiju are often known for causing chaos and demolishing cities, so the first story might revolve around this idea of destruction and the attempts to stop it.
The 'kaiju first story' might be referring to the initial narrative created around kaiju. It could involve the first time such a concept was introduced in a movie, book or some other form of media. For example, if we think about Godzilla, the first story might have been about a prehistoric sea monster awakened by nuclear tests, which then began to attack cities. This type of story set the foundation for the entire kaiju genre.
I'm not sure specifically what the 'kid kaiju story' is about as there could be many different interpretations. It might be a story about a young kaiju (a giant monster in Japanese pop culture), perhaps its adventures or how it grows up in a world full of challenges and other strange creatures.
Start with a unique concept for your kaiju. Think about its appearance, powers, and origin. Build an interesting world around it. Also, focus on creating engaging characters who interact with the kaiju in compelling ways.
Well, it's a story that likely involves a girl in some way related to kaiju. Maybe she has an encounter with kaiju or has some special connection to them. Without more specific information, it's hard to say exactly.
Since we don't have much info, we can assume the main character is the girl. There might also be other characters like her family, friends, or people she meets during her kaiju - related adventures.
Since I don't know the exact 'kid kaiju story', it could have a young kaiju as the main character. Maybe there's also a brave kid who tries to understand the kaiju and help it.