Ken had no intention to let his children try out the effects of the artifice without verifying everything himself first. This was why he had used Merik as a test subject on the way back from the Duke's towers.
But that was only one part of his true motive behind that 'prank'. The other part was that he wanted to delve deep into Merik's mind and have a taste of this young noble's true self. Since Merik wanted to befriend him so badly, Ken had no qualms in evaluating his character this way. He wasn't exactly a beacon of ethical values, he had never been that.
