One thing that strikes me about the photo is that the girl's eyes aren't fully closed. If you expand the photo, you'll see they're open a sliver. That's not the normal way you keep your eyes at rest. Either they're closed or they're open, not almost-but-not-quite closed. Also, her hands are in a very unnatural position, very rigid.

One sign of photos of dead people is that their face will be sharper than the faces of the living, because you had to remain still for a long time and the living couldn't help but move slightly during that time. You'll see that some of the other kids in the photo are in soft focus, likely as a result of micro movements. However, I wouldn't say in this case that the little girl on the left looks notably sharper in focus than the others. So basically, I can't commit to either living or dead, I'm just pointing out some signs either way. However, an image google search has turned up this photo in numerous articles about Victorian post-mortem photography, in legitimate journals that have fact-checking.

Absolutely post mortem. The history of Post mortem photography during the Victorian Era is well documented, and a large store of images are privately held by The Burns Archive

I searched online for some book links for you, but could only find amazon links, so I haven't linked them.

Beyond the Dark Veil: Post Mortem & Mourning Photography from The Thanatos Archive - Jack Mort

Seizing the Light: A Social History of Photography - Robert Hirsch

Hope this helps.