If you have a while to go before your trip, a good place to stock up on good pins just for trading is ebay. A lot of sellers sell big pin lots for cheaper than you could get them in the parks. That way they could have a lanyard and then some full before you leave for your trip.
Once you get there, any cast member with a lanyard will trade pins. The only thing is cast members wearing a green lanyard only trade with children under 12. Chldren can also trade with the other cast members, it's just the green lanyards are excusively for children.
Also if you keep your eyes peeled, the resorts sometimes have pin trading nights where they'll have special events for children and in the parks they sometimes have pin trading games (I know the Mouse Gears shop at epcot has these periodically).
Hope this helps, and let me know if you have any more questions.
"There is more treasure in books than in all the pirate's loot on Treasure Island."