Not as a number, but as a unit it actually exists. It is called a "mort" (from mortality). In that sense one mort is "so much, you'll die".

Though, the commonly used scale is a mort * 10 ^ -6. https://en.wikipedia.org/wiki/Micromort

Funnily enough, there was a short story about the killion published in the New Yorker in 1982: "The killion, as every mathematician knows, is a number so big it can kill you." [1]

[1] https://www.newyorker.com/magazine/1982/09/06/the-killion

A great opportunity to begin teaching your son some set theory until he understands inaccessible cardinals!

Wait until he learns about “Killing fields” which are vector fields whose flow preserves distance and named after Wilhelm Killing

a true mathematician in the making