I generally dislike Piers Anthony and particularly disrecommend him in the case of an elementary school girl. He has some interesting content. I own some of his books, and have read many more. But he wraps up his worldbuilding concepts and his plot notions in repulsive sexism (against both sexes) and barely passable writing/characterization and gratuitous Author Appeal gimmicks.
I would go even further. I don't think that he has any interesting content, either. What he has is a lot of puns. That's enough for a stand-up comic routine, but not enough for a book.
I bought my niece a Kindle that just arrived and I'm about to load it up with books to give it to her tomorrow for her birthday. I've decided to be a sneaky uncle and include good books that can teach better abilities to think or at least to consider science cool and interesting. She is currently in the 4th Grade with 5th coming after the Summer.
She reads basically at her own grade level so while I'm open to stuffing the Kindle with books to be read when she's ready, I'd like to focus on giving her books she can read now. Ender's Game will be on there most likely. Game of Thrones will not.
What books would you give a youngling? Her interests currently trend toward the young mystery section, Hardy Boys and the like, but in my experience she is very open to trying new books with particular interest in YA fantasy but not much interest in Sci Fi (if I'm doing any other optimizing this year, I'll try to change her opinion on Sci Fi).