I've found Scratch to be very accessible for kids starting around 7. http://scratch.mit.edu/

The visual programming language was used by Lego for the Mindstorm platform, as well as others. I believe there is an Arduino front end too.

Scratch TNG is a 3d version of Scratch tilted to simulation.

Alice, Storytelling alice, and Alice 3 are all nice. Alice3 has an onion-skinning mode where the visual programming language can be peeled down to the underlaying Java code in a series of discrete abstraction steps.

Unity3d, recently made free, would be another step along. A non programmer can get somethings to happen pretty easily. Underneath you are scripting in a javascript variant, python, or C.

my son, now 10, as been programming ( the most silly and non sensible ) games in Scratch for a couple of years.

I encourage him to send them as birthday presents.

It's readily 'get' able, and their site is full of examples created by other kids in this age range.

There is a 3d mob world version as well. (!)