When we went down last in Oct 2006, they gave them to my 2 year old twins when we checked in at the WL. They gave the 2 older boys pins also, even though it wasn't their first trip, just so they weren't left out!! I didn't even have to ask about it, though I didn't even know the pins existed. Never saw them when the older 2 went down for their first trips.
When you make your reservation or call to confirm it a few days before arrival just let them know it is her first trip and when you check in they should have the pin ready for you. That is what happened this past Jan. for my DD's birthday! Hope you make many magical memories to last a lifetime!
when you check in at your resort and tell them it is your first visit they can give you one. also as others have said you can also get it from city hall in the magic kingdom. we got several cool and different ones there.