I made a cake for my DD for her 4th birthday. If I recall, I made it all the night before, and just carefully set it into a box and sort of draped a dishtowel over the head so DD would not see it. I put it on the floor of the car (or if it was on the seat, I may have put the seat belt around the box). It was open, but with the dishtowel over it.
This is the cake I made:
I baked it in a bowl, but then had to make more cake (maybe I did a bundt cake? I can't remember) because I did use an actual Barbie doll. I knew that DD would not accept a cake topper in this instance. I bought the Cinderella Barbie and then I just kind of did my own thing with the cake.
I wrapped Barbie from the waist down in saran wrap, and I used a paper towel tube to hollow out the inside of the cake. Barbie fit in there nicely. I decorated the whole thing with buttercream and a star tip, I did a crumb coat (well, tried to but I suck at decorating and so the crumb coat didn't really work). I did set it in the garage overnight (her b-day is in December, so it was similar to refrigerating). We did have plenty of cake for everyone, adults and kids, but I don't remember how many we had.
I just remembered that I used
this thread for some instruction as well. Good luck, feel free to PM if you want any more info. I will do my best to remember!
ETA, I think we also used a cupcake for the candles because we were afraid of setting Barbie's hair on fire