I actually have three sons. My godson is currently a 5th grader. He is very bright and just started his endeavor in math competitions. His team will compete in the Washington State Math Olympiad. I just dug out the training materials for him.
Here are problems from the past.
1999- now http://www.wsmc.net/olympiad/
More study materials at http://www.wsmc.net/olympiad/study.htm
It has been ten years since my older son took his first math competition at WSMO. The journey has been incredibly rewarding. I am glad that I will get to enjoy it all over again. Best wishes to the young and fearless child!