I would venture to guess the mind of a child is just what it is...."the mind of a child". In fact, they couldn't care less about outside influences, that does not infect them in the now reality. They would much prefer to go outside, and play, than to worry about what they consider grown up business, and responsibility.

To think "children" would worry about a Planet X is grasping for straws, when in reality their parents are the ones who are able to indoctrinate, and scare them.