Originally Posted by Yapyap
I haven't read the Enola Holmes series but while I adore Skulduggery Pleasant and have enjoyed Artemis Fowl, and would otherwise wholeheartedly recommend them for any kid of that age, they're possibly not well suited for someone who has sleep issues and gets anxiety from books with too much excitement (going by Order of the Phoenix being too much). Same goes for other series I'd mention otherwise, like Rick Riordan's Percy Jackson or Kane Chronicles.
I'm trying hard to come up with something that hasn't been mentioned yet, but the majority of books I've read that are aimed at that age group tend to go for "action-packed and exciting".
Oh... how about the Underland Chronicles by Suzanne Collins? I've only read the first one (Gregor the Overlander), admittedly, but found it adventurous while well below the later HP books in the scariness or anxiety-inducing excitement levels. Another possibility might be the Ember series by Jeanne DuPrau, starting with City of Ember.
Enola Holmes is Sherlock's younger sister, 12 or 13 I think. Very nice to read and reinforces the idea that children are far more mature and capable than we often see them.,