I'm not sure if they can do that in Miami and Indian Wells simply because they don't get a lot of volunteers. Spring break for school age children is a different time than spring break for the college students, and I know that IW has had problems in the past covering all of the courts. I was there this year, didn't notice anyone that stood out. Actually, some of their ball kids were impressive with their speed and professionalism. LA has tryouts each year, but it's a summer tournament and many more children are available to work there.
Most, if not all of the LA kids, are ranked tennis players, some of them very, very good. I heard a few of them talking about 'Kei is so good, etc' and that they had the same coach as him.