Perhaps also, "Innumeracy", by John Allen Paulos. Its a great book primarily intended to discuss society's lack of understanding of mathematics, but if nothing else, is a great introduction to probability, and really gets the reader going, knowing that he/she can use math to analyze and predict real world situations and outcomes. Easily understandable by the 7th grader.

http://www.amazon.com/exec/obidos/ASIN/0809058405/qid=1026946291/sr=8-1/ref=sr_8_1/002-9117493-4686408