Khadgar42: Well, truth to be told, if you are a good uncle and take your responsibilities seriously you could play any game they like.
As long as you don't leave them alone with that.
You could even play Manhunt or Amnesia together with them if you are responsible and capable enough to tell them the difference between good and bad, right and wrong and CGA Tomato Ketchup and real blood.
I bet no grown up ever got nightmares from playing games like Amnesia or watching horror movies, right? And if they did, it was probably just because they had a bad education and didn't know right from wrong. ;)
The old Disney platformers (
Lion King,
Jungle Book,
Aladdin) seem pretty child-friendly to me, but they're probably hard to find these days and would need DOSBOX to run ...
How about
World of Goo,
or [url=http://www.youtube.com/watch?v=QWU0YTiNNgI]Botanicula? Depending on how skilled they are, maybe also Psychonauts, Beyond Good and Evil or
Aquaria?
Also, games where you can customize your own character and play around with them however you like. I know some kids like to play Neverwinter Nights and there are a few kid-friendly modules for it, but it's a bit outdated regarding the graphics. You'd have to know how to mod it and maybe play it online with her, because the original game is not that interesting for an 8-year-olf girl, I think.