Usually in larger cities, there will be areas where "homeless" people come up to your car when it's stopped in traffic and just start cleaning your windshield, even if you are yelling at them to go away, for which they expect payment. There have been news reports of things getting a bit heated.
Regarding the begging in front of the grocery store, I don't like it either. When DD was in school, they sold Krispy Kreme doughnuts on Fridays, sold candy bars, coupon books for local stores and restaurants, worked concessions at school functions, etc. They didn't stand around in front of grocery stores begging for money.
If a homeless looking person did this, the police would be called. I have personally seen this happen with an older couple, one of whom was in a wheelchair, and they were carrying a lot of stuff with them so I assume they were homeless. Obviously, I don't know the whole story and they may have been con artists, harassing the customers, or the store just didn't want those type of people hanging around their storefront.
Let's be honest here, what the kids are doing is panhandling, nothing more, nothing less, and it's illegal, regardless of what cause they are representing.