The Purple Kangaroo

When I was little I sometimes had nightmares about a purple kangaroo who wielded lightning from their hands.
Another version of this distraction tactic is "don't think of the white polar bear" which is considerably more boring.
On an interesting note, because of my experience with this as a child, if I notice a kid starting to become too worried about the consequences of a purple kangaroo, I will add in the White Polar Bear. And if they still looked concerned I turn it into a whole skit about different coloured animals not to think about, and just keep adding more and more onto the list until the child starts adding their own, at which point it becomes a game.