[Avodah] Buying minim from a child

[Ben Waxman via Avodah]

When I was in yeshiva, we were told to refrain from buying any of the 
four minim from a child. If the product was stolen, a child can't take 
possession via shinu reshut or ma'aseh. Therefore the theft falls on the 
buyer and his minim are pasul.

Thinking about during my jog, I had a few questions:

1) Is this a real chashash or simply are we trying to cover all halachik 
corners? Meaning, if I do buy from a child do I really have anything to 
worry about? And if it isn't a real concern, then why am I not being 
oveir on "chosheid b'kesheirim"?
2) If it is a real chashash (the presence of stolen minim in the market) 
than why buy from an adult? Buying stolen goods is also a sin. At the 
very least, one can only buy from a place with a heksher.
3) Or can one? Can we assume that the kashrut agencies really check were 
their minim come from?
4) Who determines if there is a significant presence of stolen goods in 
the market every year?  I assume that the amount of stolen goods has to 
be above a certain minimum for there to be a real issue?

Ben