Thanks all!
ForeverAlice-
I used a doll cake pan for the dress/cake portion. I bought
this one but Wilton makes one as well. The one I bought is a bit larger and I think the dress shape is a little nicer. If you use the pan I bought, use 2 full cake mixes to fill it. It will take a VERY long time to bake, but be patient. It will get there eventually.
The other key piece is the dolls. The "Sparkle" line of dolls is perfect because the upper portion of their costume is molded/painted onto the dolls instead of fabric. Both Anna and Elsa still have fabric capes but you can put those on after the icing is dry. This way their upper bodies are already decorated and you can put the icing right up to the bottom of their bodices and it just washes right off. The problem is that these dolls (like all Frozen merchandise) are hard to come by.
Amazon had Elsa in stock last night for $14.99 but they've sold out already and its back to scalpers-currently $45 on Amazon third party. For me, we happened to already have Elsa so I only had to buy Anna but I did end up paying a scalper $33 for her. Though looks like she's more like $60 at the moment on Amazon. Cheaper on ebay I think. Grrr.... Anyway, here are links to the exact dolls on Amazon.
Elsa
Anna
Once the cake was done, I did have to hollow out the center a bit, then removed the fabric skirts from the dolls and wrapped their lower legs in plastic wrap and stuck them in the cake. I then also wrapped the upper bodies in foil to keep icing out of their hair etc while I worked.
From there, I just made up some buttercream and colored it appropriately for each skirt (using Wilton and Duff gel colors). For Anna, more colored icing for the decorations (though I admit I bought pre colored Wilton stuff for most of that). For Elsa, I used the Pearl Wilton Color spray to give a nice sheen to her skirt- this was definitely a worthwhile touch. I also used some of their clear cake sparkles around the lower edge of her skirt then made a few swirls with the edible pearl decorations. Elsa was definitely easier than piping on all the details on Anna!
Once its time to cut the cake, just lift the dolls out of the center, clean them up and hand them over to the birthday girl!