Answer: "...going to walk on a bridge taller than the entire city".
Explanation:
The adjective "naive" best describes a state of being innocent or simple, and displaying no knowledge or experience about something, which is typical of children and very young people.
I think the narrator was female because of the statement "...putting on our finest dresses", so I will use "she" to describe the narrator. She was clearly excited to go on the tour to the newly constructed bridge, but from her narrative, what thrilled her the most was the fact that the bridge was taller than the entire city, and she would be walking "high up in the air" with her family. Her young and naive mind had a picture of a the city as everything on the ground, below the bridge, including the swarm of boats underneath. Of course, the bridge was part of what made up the entire city as opposed to what she had in mind.
Also, the statement "...something called physics" in the second paragraph also buttresses her naivety. She had no idea what the word "physics" meant, she just heard people talking about it and reported what she heard.