You might want to take a look at Logo.
It's a programming language that was developed to introduce kids to the world of logic and algorithms. It is quite powerful but still not too complicated.... I don't have kids of my own, so I can't really say if Logo is too complex for a 6yr old, but that shoudn't keep you from at least looking at it.
I'm sure you have heard of Logo and turtle-graphics which is kinda like the DirectX interface you want to code.

If you insist on writing it all by yourself, i wouldn't worry too much about long-term effects... what is really important about programming is understanding the logic behind it. And if she takes her first steps in your customized programming language, switching over to Pascal or whatever will be a breeze for her.
Visualization is a good way to introduce kids to the idea of programming, as long as you don't exaggerate it.