I would recommend Blitz3d...like some of the others did. It's simple...it comes with it's own IDE and uses a bastardized form of BASIC. Has quick return on investment so I suppose it would be a good choice for those with short attention spans (kids). There is a good layer of abstraction between the underlying graphics API (old DirectX) and the language...so at the expense of flexibility you can still achieve some really cool effects and not have to dive into a shader language (HLSL in that case) or linear algebra. After a few simple games he'll realize that Blitz3d does not scale very well at all to more complex projects...and you can get him started on something more powerful like XNA4.

Seconding (or thirding or whatever number we are at) using a small game engine for this.

It doesn't sound like your goal is to get them into programming. It sounds like your goal is to give them a game engine to play with. Yes, you can use the game engines to make professional games, but for those who are inclined to make things they can serve as great little toys.

I didn't see GameSalad mentioned, it is another good one.