I'd read Alexandre Dumas' Ten Years After (in three ~800-page volumes) through at least three times cover-to-cover by age six.
Did I understand absolutely everything about the French court intrigues? I'm fairly sure I didn't, but that didn't stand in the way of utter enjoyment.
I suspect a lot of kids, especially those who start to enjoy books early and are a bit ahead of their peers in development, are very happy with much more "mature" books than adults think. Of course there's stuff around that really isn't appropriate for very young children, but for a lot of this, I think children tend to filter out the inappropriate parts by virtue of just not quite understanding everything yet, focusing on the fun and adventures and action.
It does make suggesting books for a young reader (or book lover) tricky - obviously she's well ahead of what many other six- or seven-year-olds would be reading (or having read to them), but at the same time, it's impossible to know just how much effect, say, violence would have. Some young children
are traumatised / scared by fictional violence while others are more than fine with it, for example.
Space adventures with girls seems to be one thing that's relatively lacking, still; there's plenty of kids' and YA fantasy, both contemporary and high fantasy, with girls as the heroes (and lots and lots of dystopian YA), but I'm really hard-pressed to think of many true sci fi / space adventures with girl protagonists.