>>715638607
Why would the adults finding out about Dark World be a bad thing?
Castle Town is the only safe place to do it, and it's the part they see first.
Plus, it would confirm to Noelle, potentially Berdley, and Asgore that it was real.
From there, Undyne's disappearance could be explained, the shelter could (in theory) be opened and the day would be saved.

Huzzah, hurray, and all that. But, the Knight would have a hell of a hard time being sneaky, and might accelerate their plans.