Trin,
Interestingly enough, the child in question was introduced to Percy Jackson by yours truly. That's how I know he likes Greek mythology!

Kriston,
I think the lowest grade you can register as is 3rd for the National Exam. That doesn't mean that a six year old can't take it though; my dd was six when she took it last year. The main exam is only 30 multiple choice questions based primarily on the D'Aulaires Book of Greek Myths. (I am not going to try any fancy underlining today, as I have a wrist injury from a fall on the ice last night!) I probably shouldn't be trying to use my right hand at all.

Cym,
The old D'Aulaires book is a terrific first source for kids. My kids are telling me that they recommend the new "ology" book on mythology, and Aliki's, The Gods and Goddesses of Olympus. The Friendly Guide to Mythology by Nancy Hathaway is also good, and includes myths of many different cultures. Warning: it contains lots of references to sexual union and even in one case, mastubation that results in the birth of a god.

Some of you must be familiar with the Teaching Company lectures. We are currently finishing up a very good series of TC lectures on Greek mythology. I was able to get them through my local library- check and see if your library has them as well.

take care-