DD has an Elsa costume and is getting an Anna costume (found on clearance after Christmas for $9 at a local store!) when we are on the trip. It is her "princess treat" for our first MK day. I plan to bring the costumes on our MK days in case she asks to wear one of them.
I also bought her princess-themed outfits for our two MK days. She picked out both at Target, but they've been squirreled away in the Disney trip bin in our basement, so she has forgotten about them! One is a Frozen dress that's really just a t-shirt with attached tutu skirt. The other is a princess shirt with Aurora and Belle and somebody else on it. She tried on a purple tutu to match but said she'd rather just wear shorts.
I figure it'll be easy enough to slip on the costume over her dress or shorts if she wants to wear it. Or she might just be happy in the outfits with princesses on them and not want to change. We don't do costume shoes, either - she plays with them at daycare, but I don't buy them because I'm weird about foot development. She was begging for some at Target the other day and would want to wear them all the time, especially at Disney.