I made a 2 egg batch of my 5:4:4:2 cake mix and put 2 tbsp of the mixture in each of 2 small bowls. I coloured one bowl pink and one bowl blue using colour pastes. I think I put a little too much colouring in them, but was conscious that it needed to be obvious when you bit into it.
I made some vanilla buttercream icing using pure soya spread and smeared each cooled cake with enough to cover any imperfections where the colour showed. I added a few sprinkles and they were finished. They looked just like normal little cakes and were a great little guessing game for the children.