Preheat the oven to 180°C, fan 160°C, gas 4. Grease and line the bottom of a 1lb loaf tin with the sunflower oil and greaseproof paper.
Make the sponge mix using the egg and water, to pack instructions, then stir in the bananas and cinnamon. Using an electric whisk, beat together for 2-3 minutes, then fold in the sultanas.
Pour into the loaf tin and sprinkle over the walnuts and sugar. Bake for 55 minutes until a skewer inserted into the middle of the cake comes out clean. Remove from the oven and let cool.
Chef's tip: if your child has a nut allergy, replace the walnuts with a sprinkling of porridge oats on top instead.
This recipe contains naturally occurring sugars