So, I decided to improvise. I dug through my "miscellaneous baking things" containers until I discovered:
I used the tube of glossy icing to glue on flowers, and write on the cake. It turned out rather as you'd expect, especially after the children took turns gluing on their own artistic touches.
Please, those of you who are sensitive - look away now!
Bizarrely, my children - the same children who usually only ever eat the icing on any baked goods - declared this cake to be the BEST cake ever. And gobbled it up.