Yay for gifted readers; hooray!
My son loved the Magic Tree House, Secrets of Droon, and Beast Quest series in kindergarten.
The American Girl books (not the dolls, the books) are also lovely. My son would have loved an American Boy series if such a thing existed.
If those are too basic for her, what about the Laura Ingalls Wilder books? There are one or two passages, in the whole series, that don't align well with contemporary political sensibilities, but honestly, those should be springboards for a smart, important, fascinating discussion, not a reason not to read them. Aside from that (and these passages are very brief), they're entirely age-appropriate.
Another great series might be the Wizard of Oz books -- not just the Wizard of Oz itself, the whole series.
As is the Chronicles of Narnia, which my son devoured this past summer.
Though I somehow never read them as a kid, the Enid Blyton books are also supposed to be wonderful.
For when she's just a teeny bit older, the Anne of Green Gables series is also wonderful. I think it might be a grade level or two ahead of where she is now, but content-wise, they'll give you no trouble at all.