More than reading level, I'd be concerned about capacity to cope with scary things. By the end of book 7 lots of likable characters are dead, and it gets pretty existential, with the line between life and death rather blurred.

They're not that well written on the level of prose, but the plots are gripping, and we found that once our kid was in, he wanted to finish; you might want to be sure that's OK with you before you let her start.

DeeDee