A lot of pin traders feel that scrappers are a huge problem, but honestly, the pins have no monetary value (even pins that are bought at the parks, so are definitely legitimate Disney pins, resell for next to nothing). And you have no way to know what you're getting when you trade with a cast member -- if you're getting mostly scrappers from ebay sellers, what do you think they have on their lanyards? The pins that cast members trade aren't closely scrutinized for legitimacy. Pin trading should be a fun family activity, a souvenier, an interaction with cast members -- so don't get so caught up in whether you're trading scrappers. Buy a bunch from ebay - look through the previously-referenced thread's list of "good sellers" if you're concerned and only buy from them - but still, you really don't know if you're getting "real" Disney pins. Keep what you like, knowing you're not really building something that will finance your kid's college education, and trade away.
And yes, pin trading is GREAT for an 8-year-old. Look for cast members wearing green lanyards- they ONLY trade with kids. There are pin boards - big boards with about 100 pins - in lots of places, and many of the shops have big scrapbooks filled with pins that you can look through. Ask about those. And be sure to check the lanyards of resort cast members -they don't get traded as much, so you might find good ones there. I know there are pin boards at Wilderness Lodge, Animal Kingdom, and many more places. I've been trading (with and without my kids) for 12 years, and it's always fun.