Fridge Logic: The Book of Wisdom kids have a globe in their little hut. When you take a close look at it, you notice it's a MODERN globe, with MODERN borders. What was it doing there back in the 1920s?
Fridge Logic: The Book of Wisdom kids have claimed that they will not try to cause trouble because their chances of achieving something significant are slim at best. However, when compared, the chances of achieving anything when not trying to achieve anything are ZERO, and relatively speaking, even a slim chance of success is infinitely better than no chance at all. Therefore, logically, if they see something that they feel needs correcting, their best choice of action is not to sit idly but to act however small their chances of success are.
Fridge Brilliance: They no longer needed the book, and the best way to get Huey and Dalian to leave them alone to do their work was to simply explain that they were so smart that they were beyond the desire to take over the world.
Or they just didn't care enough to try. Or they didn't want to take all the risks that come with trying to change the world.
Or, since they already knew every possible outcome of all actions from every perspective, they had no motivation to experience it firsthand. Or maybe they saw the eventual heat-death of the universe and decided it was all futile. Or maybe they discovered that we are all one consciousness experiencing itself subjectively.
The reason Christabel was needed to play the Phantom Score wasn't because the song was too complex for a normal human. Or, if it is, that wasn't the only reason. Any normal human who played a Phantom Score would risk being just as effected by it as the audience.