Kinda sudden, but has your wife been evaluated for postpartum depression? If untreated, it can go on for a very long time, and thoughts of harming the children are one of the most common symptoms. I think the stress has gotten into her, and she did what she was able to do as a surgeon. Your wife needs psychiatric help with her psychosis and later with dealing with what she's done. As for the kids, some mothers suggest colloid silver and detox, they say it helps with any problem, cures autism, grows limbs etc. Maybe worth a try?

Do they feed on humans in general or some particular part? Maybe emotions, pain or death as such? Also is it known what they feed on in the woods if there are no people there? If they didn't originate in cities and villages they have to have a primary food source that isn't humans.

I like snakes, so I can't bring myself to see a snake as evil :)

I wonder if the poor girl was eating cannabis cookies and seeing things. There was no Alfred but she was getting the munchies after a dose of cannabis and her mind kind of warped it to be a friend she needed to feed. She thought herself invisible, so she invented an equally invisible friend. Then her parents found out she was getting away in the woods to eat "extra" food, got angry and basically starved her to death. The last entries are probably not much of a help, she most likely was delusional from hunger and, knowing she was dying, she hallucinated herself being buried where she had her "safe place".

Writing in the third person could be from depersonalisation, either drugs or just from the dire condition she was in. Maybe you should ask the school stuff if she was ever talking about herself in the third person? Or maybe all of it was written in her last day when she was so bad she didn't see herself as herself...