Hi there, I love doing these cakes and a number of years ago I figured out a way to make it easier and more stable. I have posted this on another site and thought it might come in handy for you.
Also, you may want to mention to the mother that the easiest way to cut up a Wondermold cake is to cut it lengthwise down the centre, then turn these two halves down horizontally so that the un-iced part is underneath, then slice in equal portions. Most people try to cut it like a normal cake and the slices are so high, they are awkward. Also, if you board the Wondermold cake the way I have suggested, it makes it easier to serve and you will get enough portions for 20 or more.
Regardless of whether you cover it in fondant or buttercream, remember to place little ropes or strips or strings of icing in several places down the skirt so that when you ice or put fondant over them, you will get the folds hanging in the skirt for a more natural look.
The Wondermold isn't deep enough on its own for a Barbie cake, so you need an extra layer or two of cake underneath so that Barbie can go inside the cake and the cake starts at her waist. I use petal pans. A 9 inch and sometimes a larger one - up to the 15 inch size. Of course you can do it with the round or even place the Wondermold - on a dowelled sheet cake.
First of all, the top of the extra layer or layers of cake has to be iced. Also fill if you are using more than one layer as your bottom cake that the Wondermold will sit on.
I found a great way of keeping Barbie clean and also coring the cake and keeping her enclosed and stable. An empty paper towel roll - well two actually. You are best off boarding the Wondermold and also using a strong cake base for the whole cake. So you mark the centre of the underneath of the double covered boards that the Wondermold will sit on, I places them covered sides out and glue gun the two together. Then I cut a hole large enough for a paper towel inner tube to fit throughthe centre. So you work the paper towel cardboard roll straight up through the bottom or underneath side of the boarded Wondermold cake and then remove. This will core your cake. Alternately, you can just fill with icing between the Wondermold cake and the additional layer, no boards and core from the top down. I have found that this works well, undowelled too. It just makes the cakes slices a little more awkward because of the height of them. Ok, so if you boarded the Wondermold, and now you have cored it, you also need to core the layer cake this is sitting on. So you centre a board the same size as the boarded cake on the lower cake and mark off the centre of the lower cake, then core the centre again using the paper towel roll. You will want to put 5 dowels in the centre of the marked area on the lower cake around the cored hole. Place some icing sugar in the area where the Wondermold will sit on the lower cake. So now you place the cored Wondermold cake over the dowelled lower cake and line up the holes. So now you are going to take a fresh paper towel inner cardboard tube. Insert Barbie so that the tube starts at her waist - it is a tight fit - Barbie's hip's will make it a really tight fit. So you check to see if the paper towel roll when inserted with the Barbie, will be too long for the height of your lower cake and the Wondermold combined, and you slice off the correct amount from the bottom of the paper towel roll. Now cover the roll with foil or plastic wrap and reinsert the Barbie up to her waist. If the height of your combined lower levels and the Wondermold, will be much more than the enclosed Barbie in the paper towel roll, then you will want to also place a dowel inside of the enclosed Barbie/roll so that you have a centre dowel that goes though all of the layers to the bottom . Otherwise the Barbie in the roll acts as your centre dowel. So now you are ready to decorate.
I tranport these doll cakes using a moving box. I cut the front flap all the way down. Then I completely line the box with foil and tape it well. I line the bottom with that rubbery shelf liner that stops any movement - I get it from the Dollar Store. You can cut off the top flaps if the box is high enough to enclose Barbie or tape them all up in a vertical position. So you tape the front flap back in place. Then I take another piece of foil and tape it in place over the top of the opened box. When deliverd, a utility knife is used to cut the front flap that was taped, open.