See my "thank you" post to several of the "regulars" that helped us. By the end of our trip last week, we had bought our first pin backpack, and filled it up!
You can buy lanyards at the resorts or at the park, and can probably find a picture of what one looks like if you go to ebay and type "disney lanyard" in the search area.
I would suggest going to your local
Disney store and picking up a few pins. Any CM at Disney will trade you anything they have on their lanyard, so it doesn't matter what pins you are trading. If you want to attend the "more serious" trading meets, or trade with other private traders at the parks, you will want to look for some limited edition pins if possible, or something you can only get in your area (like the FDNY Mickey pin from New York, for instance).
Most private traders (I noticed) are especially nice to kids and one wonderful trader I met actually had a special kids page where kids could pick anything they wanted in exchange for whatever they had to trade. Do be careful though, because there are always (unfortunately) some sharkies. Just make sure to tell your son or daughter to check with you before making a trade (not that I really knew enough to always be too helpful...lol!). But most of all, have fun with it!!
Sharon