My boys love trading and collecting- they got hooked on our very first trip with our then 4 yr old happily trading out one pin for another with each lanyard wearing CM he met
He only had one pin to trade and he traded that one at least 6 times in one afternoon.
Don't know if they still do this but the Premiere pin shop in TL had a pin trading class for children. They tell you all about how to politely trade, which pins are tradeable and at the end they give them two pins to pick - one to keep and one to trade.
I believe the pin shop is now called "Little green men store command" as it is the exit for the new Buzz ride.
Alos, the first Sunday of the month they do Gift with Purchase pins (GWP) - spend $15 and get a free mini pin. They have certain themes /sets every few months. I love collecting these tiny pins and try to get the whole set. I have a partial set of Aladdin, partial of Peter Pan and a complete Winnie the Pooh set. We also collect the AP pins, I have a few Disney Club pins before they switched to the Visa, and I focus on Tinkerbell and holiday themed pins fo rmy own collection while the boys get whatever has good "coolness" factor .
I pick up cheap tradebles on ebay - trying to keep the cost under $3 a pin(with ship). Most pins range from $7- $15 in the parks.
Disney Store sadly no longer sells pins-- that was my other cheap source and way to feed my frenzy of collecting since we live to far away from any Disney park... Minnesota!!