Pin trading is a great way to add to your kids enjoyment at WDW. All of my kids are big fans.
Our experience has been that any Cast Members at the Theme Parks and Resorts that has a lanyard with pins will be willing to trade, assuming they aren't busy doing something else. Guests can make up to two trades per lanyard, per Cast Member, per day. When trading with
Cast Members, Guests should offer a pin that is not already displayed
on the Cast Member's lanyard.
Cast Members with green lanyards will ONLY trade with children.
I have heard that some Cast Members might have a "showcase" pin that they are not allowed to trade, but I have not run in to this issue ever.
You can find some good overview information
here, and much more detail at the
Disney Official Pin Trading site (see the "Getting Started" section for lots of FAQ answers).