If memory serves, this was in honour of some children's book event at which all attending children were to be dressed as a character from a book. Since Roald Dahl was to preside, I went dressed as Willy Wonka (and at one point I had a signed copy of The Wonderful Tale of Henry Sugar that I acquired as a result).
sophievdennis went as the Owl Who Was Afraid of the Dark and won a prize.
My mother claims I spent a lot of my childhood dressed up as things and considers this an early sign of an interest in Larp. However, as an adult, I notice the amount of work that has gone into that costume (certainly not by me). That red jacket, for instance, was cut down and modified from a cast off coat, and the cane was custom made with silver foil decorating the handle. I am not convinced, therefore, that an interest in dressing up and making costume simply mysteriously appeared out of nowhere in our family on the appearance of the two of us on the scene.