The central idea of CJ Dennis's poem "The Circus" is carpe diem.  Carpe diem means "seize the day."  Ben Franklin said it like this: "Never leave that till tomorrow which you can do today."  

"The Circus" is bubbling with excitement.  The speaker can hardly contain his joy at having the circus in town.  It seems as though the speaker has already seen the circus, because his questions appear to be directed at random listeners about the events he saw and knows are at the circus.  He is so excited about what he has seen that he is asking everybody else if they have seen it too.  

The poem conveys the idea that if you haven't seen the circus yet, then what are you waiting for?  Go there!  Do it now!  See it now!  The exclamation points help the author sell his adamant feelings that the listeners should seize the day and go to the circus now.  Dennis uses 14 exclamation points in 12 lines of poetry.  It's exhausting to read (like a lot of text messages can be). 

If you forced me to pick another central idea, it would be that going to a circus is some of the greatest fun a person can ever have.  Why is it so fun?  As the speaker indicates, it has elephants, clowns, acrobats, horses, tumbling men, jumping dogs, jugglers, and all kinds of great stuff.