You guys don't know my four year old.

She'll make it, probably with half a year to spare.
She decided she'd rather save all her money for "extra Disney time" rather than an extra princess dress. I don't think there's anything wrong with that.
As for them paying for the family vacation, we're taking them on the Fantasy for a week following these bonus days at WDW. I don't think they're hurting for vacation.
They're more excited saving for this than anything they've saved for yet, for what it's worth. Mostly I'm proud that they get the concept of saving and that I have two little budget conscious mouseketeers!