From the brief description, this sounds to be quite basic. Looking forward to hearing if Terence has treated the explanations differently. :-)
G.H. Hardy and E. M. Wright "An Introduction to the Theory of Numbers"It should be according to Tao's own comment at the bottom of the blog:
"This book is for a general audience, without necessarily having a college-level math education. It is aimed more at adults than at children, but some children with an interest in mathematics may be able to get something of it."