Yeah but if you let a lemonade stand slide, then what keeps an adult from setting up a business on the sidewalk?
What if you change the law so that only kids can sell lemonade without a permit, but then assholes start churning out lemonade stands using child labor?
Honestly though, a Lemonade stand is usually on someones private property. Selling water in the National Mall is on someone elses property, plus it's potentially taking sales from a business that leases the space. The property owner needs to protect their business interests, which are those paying a lease.