I must be a cold-hearted parent, but I can easily say no to those cute girls, with their little fake frowny faces put on for the camera.

(Visualize me standing there with arms crossed and an eyebrow raised, saying, "Nope, ain't gonna work.")
First off, their own parents must have had other reasons to fly them all the way from Scotland
besides buying an Elsa dress.
If they didn't, then their parents need a kick in the butt. Especially as the price of airfare would have easily covered several overpriced EBay dresses, and a ton of beautiful hand-made Etsy ones.
Secondly, girls this age are more than old enough to realize that just because a store sells a particular popular item, that doesn't mean they're in stock all the time. So, if you're going to travel any distance to get to the store, you call ahead and try to have them put one aside for you, and if they won't, then you realize you're taking your chances like everyone else.
And if you're crossing oceans to get there... See point one.