Hi Michael,
You can buy authorized Disney pins at the theme parks, Disney Resorts, Disney stores, and on DisneyStore.com. There are also many guests that buy resale Disney pins on sites such as eBay or Amazon. You can often find substantial savings and great value from online resellers.
While most resellers offer legit Disney pins, there occasionally unscrupulous vendors who sell fake pins. When you buy online, you do take a bit of a gamble. Be sure to look at the seller ratings if you decide to go that route. You can look at the
Disney Pin Trading Guidelines for ways to spot legitimate pins. Frankly, many cast members themselves will be only somewhat educated about real/fake pins, but if a fake pin is offer and the cast member is aware, then yes, your child could be refused a trade.
Good luck with your planning. Please let us know if you have additional questions.