Here is Minchin as a guest on Conan from two years ago. performing the real version of the song. It comes in about the 3:30 mark, so you can jump to it -- but the interview is quite entertaining. He talks about his edgy comedy and making the transition to writing a musical about children -- as well as upcoming projects that includes a stage musical version of the movie, Groundhog Day. Since this interview is two years ago, Groundhog Day opened on London's West End and did very well, with solid reviews and is scheduled to open on Broadway.
When he performs "When I Grow Up" song from Matilda, he's helped out by three of the touring Matilda's from the Los Angeles touring production.