Someone here said they bring 5-15 pins to trade -- that's a minimum I trade PER DAY! It gets addictive. Try to get your kids to think about a theme they'd like to collect -- a certain character, rides they have gone on, holidays, etc. You can go to pinpics.com and look at pins just to get an idea of what there is out there. You can also go to odpt.com (the official Disney pin trading site) and see what will be sold at the parks when you go. I've always found ebay to be a great site to buy lots of pins -- I usually aim for less than $2 a pin for traders.
Have fun!
Erin