>>120339They could have just imprisoned a bunch of other kids and caretakers with him. The kids would be just as ignorant of their imprisonment as he, while the caretakers would know better than to speak of things they shouldn't.
It breaks their human rights, but then, he is a world ending threat.
Alternatively, if they really trust that he can only kill by sight, weld the door shut for good and leave him alone in there. If they want to be humane, keep him company via robots and feed him via meals served conveyor belt sushi style.