My nephew recently celebrated his 5th birthday with a pool party. so of course we made him a pool party cake, and OF COURSE it was a Lego pool party!
I think he liked it!
there are tutorials all over the web for the Jello pool cake. I made a classic white cake in a 9 x 11 glass pane, and cut out a shallow pool. I iced the whole thing with cream cheese frosting and then lined the pool with white fondant. The 'tiles' are also fondant. I think it turned out well and of course if the birthday boy is pleased that is the most important thing!