In a true entrepreneur spirit (and with the age-old wisdom of "location, location, location"), the kids acknowledge they had picked their spot just down from the filming location in hopes of capturing some of those tourist dollars. Hollywood actor tourist dollars, of course. Carter Fontaine and his brothers in business Chandler and Chase were asking a mere twenty-five-cents per glass; the boys' mother had at first offered the lemonade to Clooney for free. (Ah, the perks of stardom.)
Clooney, however, wouldn't hear of it. Apparently having no cash on him, he sent an assistant around with a $20 bill, and with the instructions for the boys to keep the change.
The Carter Brothers also know the benefits of advertising: their stand now boasts a large "George Clooney Was Here!" sign. I'll be looking for them on "The Apprentice" in 2027.