At 6 and 4, I would take the number of pins you have that you are willing to trade and divide it by the number of days you will be at WDW. Then cut that in half, and that's the number of pins each one of your kids have available to trade each day.
You can, of course, be flexible when they find a pin they just *have* to have and they've already done their trading for the day, or if they look around all day and don't see anything they want to trade for.
I do think that having a guideline number of pins to trade per day will help you mete the trader pins out over your entire trip, so the activity will be enjoyable for the kids over the length of your trip. This will keep them from getting so excited early on in the trip that they trade all the traders away and then they will be bugging you to buy more so they can keep trading.
There are some pin trading specific boards out there on the internet where you may find a truckload more information and pin traders who can give you all sorts of information. I agree with the previous posters that you do want to be careful of the authenticity of the pins on eBay (not saying all the pins there are fakes, but I know that there are *some* unscrupulous sellers there) and that you may want to look into getting some extra pin backs that are better at keeping the pins on lanyards/clothing than the Mickey head backs the pins come with. You can find them in some craft stores, or online by searching for "security pin backs", "safety pin backs" or "locking pin backs".
I hope your kids have a blast trading. It's been really great for me to see my daughter (almost 5 now, 3 when she started trading) interacting with the cast members with increasing confidence over the period of time she's been trading. You may also learn a few things about your kids when they trade pins! I had no idea she liked some of the characters she does until she picked out pins with those characters on them while she was pin trading.