Ok, normally I'm not a spend-a-fortune-on-Disney type, but Tink costume would be so stinkin' cute! My dd was Tink for Halloween when she was 2 and I just can't get enough of looking at those pictures. In fact, when I'm out of hock with these points and can have an avatar again, I'll put her in ther Tink costume on my profile.
I'd bring maybe one costume so she feels like one of the big girls and then bring an outift of comfy clothes to change into. If you are totally set on having her in costume, you may want to consider is checking Etsy for some princess-inspired sundresses that would be more comfortable for your little one but would still have some princess flair. I am planning to make a Sleeping Beauty inspired sundress for my DD's trip to "Cinderella's Castle".