Piggy's glasses are important because they enable Ralph's group to light a signal fire that can help them get rescued. The glasses are later used by Jack's group to light fires for having pig roasts. Thats why Jack wants them
We weren't quite sure what it all meant, except that for now, my sister Rachel and I were putting on our finest dresses and the shoes Mother had polished twice.
Being naive simply means that one lacks experience and knowledge or critical thinking regarding the world around him.
The narrator, being young and naive, can not fully understand all the difficulties that surrounded building of the bridge. All he can understand is that people said it couldn't be done, but they've done it and he sees that as something special, out of this world experience.
He shares other people's happiness and is satisfied that he and his family are able to walk on that special bridge without being able to comprehend all the effort and obstacles and sacrifices endured for the bridge, that represents national pride, to be built.