Greetings from Australia. I came across this website last week which gave me some fantastic ideas for my daughter's 7th birthday pool party cake. I took a few ideas from different cakes and came up with this Pool Party Jelly Cake:
I baked a 22cm x 30cm slab cake using 2 packet mixes. I drew the shape of the "pool" onto baking paper to use as a template to cut out one section of the cake when cool. As I only used one slab, I didn't cut down too far - about 2cm deep. I then put the baking paper into the cut out section to hold the jelly - this stops the jelly seeping through the rest of the cake and making it soggy.
I made up one packet of blue jelly and let cool in the fridge. I iced the entire cake with just a basic butter icing tinted green(1.5 cups of icing mixture and 60g butter) which hardens. It is very hot here in Queensland, Australia so you don't want soft icing which melts.
I then tinted the dessicated coconut, by putting the coconut in a bowl, adding the colouring and then mixing it around with my fingers. I spread this over the cake for the "grass".
After the jelly had been in the fridge for 1.5 hrs and was still able to be poured, I poured it into the cut out pool(be careful not to overfill it or it will run all over the rest of the cake. I decorated the top with Polly Pocket Cruise Ship set figurines and umbrella straws and then chilled the entire cake in the fridge overnight to set the jelly.
The cake was a hit with the girls - they loved it, especially the jelly. With the baking paper lining the pool, it was easy to scoop out the jelly to serve with a bit of cake.