It's easy to say that the kids are not spending money. That is, until you kick them out and discover the difference in profits. And, besides, even a kid who doesn't spend a dime will generate income for the businesses.
Think about it. If you are looking to buy something, are you going to go to the place that is always empty, or are you going to go somewhere that looks like the place everyone wants to be?
Even if the kids aren't spending money, they provide the image of popular shops. And, the image of popularity is the best sales tool available.
Think about it... Why do clubs let good looking girls in for free? To get guys to think it's the place to be. No good looking girls, no guys. No guys, you have an empty club. And, who wants to go to an empty club?
The same thing holds true for malls. Why would I want to go somewhere to shop if it doesn't appear than anyone else wants to be there?
Kids make places look busy. Busy places look like the place to be. The place to be, is where everyone will go to spend their money.