Where do the children go to begin their mission?

The children begin their mission at The Learning Institute for the Very Enlightened. This setting serves as the primary location where they come together, initially under the guise of being students. It is here that they uncover the true intentions of the institute, which allows them to form their plan to thwart the schemes of Mr. Curtain. The Learning Institute is a pivotal environment in the story, providing the children with both challenges and opportunities to work together and use their unique talents. This location is fundamental to the development of their characters and the unfolding of the plot, making it the correct answer for their mission's starting point.