My favorite books by Gardner, and these are great introductions to his lighthearted approach to math, are the pair Aha! Insight [1] and Aha! Gotcha. These are probably the least rigorous works he made, but extremely fun and still tackle interesting subjects (in other words, great for kids). If you want something longer form or less approachable, he was a very prolific author so any of his other dozens of books are great too.

[1] http://www.amazon.com/Aha-Insight-Martin-Gardner/dp/07167101...