The children did. But the staff took it away, and they used that time to change the prop
I would be jabbing my hand before going on stage! I would be way too paranoid!
I mean, yeah. But they're children.
And,... spoiler alert.
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The staff is actually the culprit, and the knife isn't switched. There's a button that made the blade don't get inside. Totally dangerous.
But the princess is still alive thankfully.
SHE IS????
Yes, someone link the novel in the comments
The knife was already on stage and she couldn't exactly test it on stage in front of everyone.
It does seem extra dumb though, because surely the mind controlled murderer would bring their own knife rather than hope they find one in the victim's bedroom.
WHY DOES NO ONE EVER TEST THE OBJECTS RIGHT BEFORE THEY USE THEM?!! (⊙…⊙ )