Shirley's conscious of all of the exciting new things America has to offer. She's always marveling at things, ranging from the big—like the Brooklyn Bridge (how do those ropes hold the bridge up, after all?)—to the small and simple, like things in her apartment. For example, Shirley exclaims over her new sofa bed:
When she opened them again, there stood a giant bed fit for an emperor. Shirley threw herself on the mattress and lolled about like a fish tossed back to the sea. "How did you do it, Father? How?"
But before he could say a word, she shouted, "I know. It's just another wonderful engine made in America." (8.21-22)
Her new home is full of thrilling new opportunities, and Shirley embraces them, both because of their novelty and because she wants to fit in with her friends. She wonders at the way baseball works, but falls in love with stickball and the Dodgers because that's what her friends like.
Similarly, her perceptions of people are rooted in her vivid imagination. When meeting someone, Shirley doesn't just describe what he or she looks like. She paints an animated portrait that is a product of a child's vivid and astute imagination. For example, after Joseph invites her to play, she sees the following:
His face was pure white, as if his mother had powdered her baby on the wrong end. But to Shirley, at this moment, he was the handsomest boy in all of Brooklyn. (4.11)
Only a kid would think of something so funny and imaginative, using their unadulterated creative powers to make their normal classmates something strange and different.
In short, since Shirley is in an entirely new country in this story—and since she embraces the experience so fully—the tone is filled with wonder and imagination. You know, just like Shirley is.