Don’t know if this is what you are looking for, but you can actually make a cake that will be the princess dress for your Barbie, and insert a real Barbie in the middle, so the torso is the real Barbie, and the cake is the dress (did not invent anything new here, it was really popular in the 80’s and is slowly coming back, at least where I live). I have done one which I am actually pretty happy with. I have used a recipe that I have had for years, which is not a cake from Rose. Recipe calls for two nine inch pans (the small ones, of which you cannot take the side off). I have baked it in a detachable 9 inch mold and have put all the recipe in it and it came out fine. I have also done one more batch of the recipe which I have cooked in a stainless steel salad bowl that had just the shape I needed for the top of the dress and it also came out fine. Then, put them on top of one another and carve a bit if you need to (I had just tiny parts to take off to have an almost perfect dress shape). I have used cream cheese frosting and then covered with chocolate that you melt to make it look like the dress is really in fabric.
My cake was also for a 5 year-old, who wanted not Barbie but Aurora (from Walt Disney).
Good luck with your cake,