I wasn’t going to comment, but since I’m back in the thread I’ll say that I wasn’t researching at Wikipedia, just looking for a handy quote. I know my Nancy Drew.
As for the rest, and this is what brought me back, is the girl issue. It’s, I’ll say it, biased, to dismiss Nancy Drew and suggest only boy detectives in her place. That is what made Nancy Drew awesome; a girl with agency! A girl that grown men listened to! Girls need to read books like that and Nancy while not the first (I really do like kids lit from the early 20th century) was easily the most compelling.
If you’re looking for a girl detective more grounded in reality, I’d suggest the early Trixie Belden books. Again, there’s the unfortunate updating and as with all series, they deteriorated and became less credible. Still, for the young girl in your life, I’d go with that. And I liked the Three Investigators; Encyclopedia Brown is too much of a gimmick.