You can get them from the front desk at your resort, or at Guest Relations in any of the parks.
I wore one on my recent (first) visit for the first few days of my trip. I did get a lot of warm welcomes from cast members, which was nice (and got upgraded to concierge level at Contemporary, but that's probably unrelated!), but oddly enough I was more often approached by other visitors who marveled at the fact that I was in my 20s and only just making it to WDW for the first time. After the fourth such questioning I decided to stop wearing it. But your DD should be fine in that regard.