In many Seuss books, there's a clear divide between children and adults, a theme Seuss really establishes in his first book for kids, And to Think That I Saw it on Mulberry Street. The book explores the difference between how kids perceive the world and how adults do. Marco and his dad can look at the exact same thing, but where his dad will see a plain old horse and wagon, Marco sees a parade of zany characters and confetti bombs. As Marco puts it:

But when I tell him where I've been
And what I think I've seen,
He looks at me and sternly says,
"Your eyesight's much too keen." (5-8) 

In other words, even though Marco thinks he's seen certain things, his father rejects these notions outright. He thinks that Marco's eyesight is wrong and that he's not observing the real world.

Along this line, the book explores the wonder of childhood and how easily it is lost. The world that Marco creates is nothing short of extraordinary; for instance:

With a roar of its motor an airplane appears
And dumps out confetti while everyone cheers. (94-95)

Marco revels in the excitement of it all and is even eager to share the whole story with his dad. But when he sees his dad, he realizes that as an adult, he won't understand. So even though his dad seems interested in what Marco has to share, he doesn't actually get it. When Marco gets home, he asks him:

Dad looked at me sharply and pulled at his chin.
He frowned at me sternly from there in his seat,
"Was there nothing to look at… no people to greet?
Did nothing excite you or make your heart beat?" (117-120)

But even as Marco's dad is asking him about the exciting things he's seen, he's doing so in a disapproving and stern manner. He doesn't really want to hear about Marco's imagination—he wants him to be taken with the world, but in a way that isn't fantastical, that makes sense to adults—and so Marco sticks to the story about the plain old horse and wagon, even though it's not really what he experienced.

In the end, Marco's dad ends up repressing his childlike imagination and joy. Instead of allowing his flights of fancy and nurturing his creativity, his father forces him to conform to the same, old boring world that everyone else lives in.

Seuss is obviously Team Kids all the way and shows in this book that it's dangerous when adults try to repress the wild and valuable imaginations that kids have. Instead of sharing in the wonder that kids experience, adults try to stop them from enjoying things and force super boring realities on them instead. What a drag.

In the end, poor Marco concedes to his dad's idea of reality and he gets embarrassed when he tells him what he actually saw:

"Nothing," I said, growing red as a beet,
"But a plain horse and wagon on Mulberry Street." (121-122)

Marco's dad has managed to make him feel embarrassed about sharing his stories with him, but that doesn't mean he's won. After all, Marco's still got that imagination on him.