I think your child's interest and involvement in pin trading will depend on a lot of things...age, how often you can go to the parks, interest in certain characters, etc.
If this is your first time trading, I wouldn't go overboard on buying pins. I still think ebay is a good idea but stick to lots of around 20 pins or so. You are less likely to get fakes or junk pins that way. In other words, you don't want to invest a lot of money in something that may not be a huge interest for your kids.
I ask how frequently you visit the parks because if it isn't something you do a lot your kids will be very distracted by attractions and everything going on to get too big into pin trading. I am a hobby pin trader myself who goes to WDW several times a year and I still find myself sometimes too distracted to trade pins.
Most people put their pins on lanyards, those cute neck straps that usually have a hanging badge or medal. Disney sells great lanyard starter kits of different characters in the $25 price range. They usually include four pins, two to trade, and two to keep. You can find those on ebay also and if it is still in the package at least you know what you are getting.
So start off small and if it takes off and they really get into it at the parks you can always buy more pins there, that's for sure.
