I think adaptation and discovery are main themes. The children in this new world need to adapt to new situations and discover new things about their world and themselves.
One possible theme could be change. As they are children of a new world, change is likely to be a big part of their story.
The short story 'Children of the New World' may center around the idea of a new beginning for the youth. It might depict how these children are the products of a world that has been reshaped by various factors like environmental changes, scientific breakthroughs, or political revolutions. Their characters could be molded by the opportunities and limitations of this new world, and the story could be a journey through their hopes, fears, and growth.
I'm not sure specifically as there could be many different stories with this title. It might be about children growing up in a new and changed world, perhaps dealing with new technologies, different social systems or environmental changes.
The theme of social inequality is likely to be present. Some children in the city may have access to all the luxuries and opportunities, while others are living in poverty and have limited access to basic necessities like food and shelter. This disparity can create a strong theme in the story. Also, the theme of hope could be there. Despite the difficulties, the children may still hold onto hope for a better future.
One main theme could be exploitation. Children were often exploited in mills, working long hours in harsh conditions. Another might be the loss of childhood. They didn't have the normal experiences of play and education. And also poverty, as their families were likely poor which led them to work in the mills in the first place.
One main theme could be imagination. Since it's about children's dreams, imagination is likely to play a big role as they dream up all sorts of scenarios.
Friendship might be a theme too. Children often create their own worlds together, and through that process, they form strong bonds with each other. So the story could be about how they collaborate and support each other in building this world.
One main theme could be discovery. Discovering new places, new ideas, or new aspects of oneself. Another might be adaptation, as characters would need to adapt to the new world they find themselves in.
One possible theme could be hope for the future. Since it's about tomorrow's children, it may express the idea that the future generation will bring positive change.
One possible theme could be transformation. Since fire is often a symbol of change, the 'children of fire' may go through some sort of personal transformation. Another theme might be the struggle between the ordinary and the extraordinary. If these children are special in some way related to fire, they may face difficulties in fitting into a normal world.
Since I don't know the exact story, it could be a group of children with different personalities. There might be a brave leader - type character, a smart and curious one, and perhaps a shy but kind - hearted child.