Ask questions. Ask the kids how much money they make off of each candy bar or box. Find out how they got to the spot where they approached you. If they say they're with a school, call up the school on your cell phone and check.
Be vigilant. If you see candy kids going door to door in your neighborhood, call the police and ask your neighbors to call too. When police realize citizens are upset about this child exploitation, they'll learn how to work with labor departments to stop it.
Don't buy from candy kids. If you must help these kids, you could consider giving them a "donation" instead. Many kids actually ask for donations, which is tantamount to organized begging.
Your local police department and your state labor department.