As I said here, I'd go for Squeakland and the famous Drive a Car example (powered by Squeak).
Smalltalk syntax is simple, which is great for children.
And later as the child evolves, he can learn more complex and even very advanced concepts that are also in Squeak (eg. programing statefull webapps with automated refactoring and automated unit tests!).
And like @cpuguru and @Rotem said, Scratch (also Squeak based) is great too.
