Cast members aren't allowed to refuse a trade for a pin that's on their lanyard. If your son or daughter sees a pin they want, all they have to do is trade one of their own pins for the one they want. Your son or daughter gets to choose which of their pins to give up, and they get to choose the specific pin they want on the CM's lanyard.
For help in identifying the pins you're looking for, go to
Pin Pics.com and use their search function. Their database includes a picture and info on just about every Disney pin ever made. You can narrow the search field in many ways, i.e. choose WDW pins and type in giraffe, or hippo, or roller coaster.
And, yes, be wary of the pin sharks that hang around at Downtown Disney and Pin Central in Epcot. Most of them are fine, but you do run across some who will take advantage of an inexperienced trader -- young or old.
Hope this is helpful.