Maybe you should imagine an ice age or a giant meteorite hitting the earth

Z: Dad, I'm afraid of the snakes under my bed.

Me: You know what to do. You need to send Rikki Tikki Tavi under there. You need a mongoose.

Z: But Rikki Tikki Tavi isn't real.

Me: Neither are the snakes under your bed.

Z (after thinking this through for a minute): Dad. I'm afraid that sabre-tooth tigers are going to sneak up on me.