Non-textbook Math book recommendation to read to my kids

When I was a child, I really liked the book The Number Devil by Hans Magnus Enzensberger.

Not really mathematics, but do check out Norton Juster's Phantom Tollbooth. It's surely age appropriate; some of the other suggestions here are a stretch, I think.

Definitely

https://en.wikipedia.org/wiki/The_Number_Devil

by Hans Magnus Enzensberger

In Italy the title is "Il Mago dei Numeri" (The Numbers Wizard) to avoid offense to Vatican Holy See Pope, I guess :)

The original title is

Der Zahlenteufel. Ein Kopfkissenbuch für alle, die Angst vor der Mathematik haben

More or less

The number devil. A pillow book for those who are afraid of mathematics

The World of Mathematics, edited by James R. Newman, 1956. I received this 4-volume set at the age of 10 as a gift, and although some of it was over my head, much of it was fully accessible to a youngster. In the 55 years since I first met this book, I've returned to it again and again. Unlike this reviewer, the book never gets old.