I got this one from a used bookseller, it's oop but if your library has it, it is really nice for younger children.
The math concepts it addresses is relativity. "How far is far?" asked the little boy. "As far as the end of your nose," said his mother, and she kissed the end of his nose.
"As far as you can walk until you get tired. As far away as the other side of the world. Even as far away as the first star that shines when the sky grows dark."
"That's a lot to think about," said the little boy, and he went out to dig a hole.
"How deep is down?" asked the little boy.
And so on.
The story threads through like this for about 20 pages (it's an OLD book and doesn't have an ISBN number or page numbers, but it was a Parent's Magazine Press Book). How high is high, how big is big, and many other questions are "discussed" with creatures and parents, it really is a lovely dialogue and enjoyable to read. Illustrations are almost like tissue-paper collage.