The cake I'm showing you was made for my granddaughter, and as she lives in London which is 300 miles from me, I have to do as much work at home as possible and struggle with huge boxes by train and then bake the cake itself at my girl's house.
As in a lot of my cakes, I do the main body of it in polystyrene, which means I can spend many hours working on it, without worrying the cake will spoil. The rock where all the characters are, is the polystyrene part, covered in sugar paste then painted over to give it some texture, then royal iced piping for the seaweed.
I made a mold of a Barbie Doll's parts, actually just the head and arms, using modelling plaster that sets quickly. The hardest thing to get right was the little crab. Oh boy, the times he nearly went sailing through the window, and Ursula's lovely double chins was a bit of a problem too. Just couldn't get her right. I also made the sea horses and put them on wires so they were suspended as if swimming.
The cake was my usual Madeira covered in sugar paste and piped seaweed again, plus some fish with bubbles piped. I was awful to my Amelia as I'd just finished it all after days on my feet, and she dashes in from playing and you know when they get over excited and they turn into little monsters? Well she'd reached that point and she walks around the cake humming and nodding , then says, "Nanny, you forgot his bab-bules". Being a cockney that's how she says bubbles. I looked and sure enough I had forgotten the bubbles on just one of the fish, and all of a sudden a very tired nanny turned into a monster too, so I answered "That's because he's dead!" Luckily she thought that was hilarious.
The cake turned out great and after a year she still has the rock part in her bedroom. Hope I've given you some ideas, especially making molds. It's very satisfying to do your own and saves a whole lot of money too.