Just to clarify a little....they will get BUTTONS that say "first visit" at check-in. (Tell them to ask if they're not offered!) There is also a "My First Visit" PIN, like you would collect on a lanyard. I don't see them in the
shop disney app right now, but you could try calling WDW Guest Merchandise or the Disney Springs pin store direct and asking if you could purchase the pins and have them shipped, if that's what you're looking for.
If it is the pin, I do also like PP's idea of a lanyard as a gift; we love ours. And/or you could buy them a few assorted pins on eBay ("lot for trading") to start trading with; just be careful that they're authentic pins (check reviews!).
Very nice of you to want to do something special for your friends' first trip