What did really happen with the children? (spoilers)
I wondered how Benigna could have killed all 5 children and incinerate their bodies (in an oven with wooden doors?!) without anyone else noticing. That is just pretty much impossible.
Let's assume the other staff did not suspect she laced the cake with poison. Wouldn't they have heard the children's screams and called a doctor? Even if the poison had no immediate effect and they didn't hear the children at night, they would've called the police in the morning, unless they panicked and decided to fabricate some story about the children being moved to another house or something. And then they burned the bodies but never even buried the remains?
Seems like a huge plot hole to me. One explanation that makes a bit of sense is that the staff did not panick but conspired to get rid of the children for whatever reason. Still, why weren't the children buried? Also, note that it is not clear the poisoning ever happened. Only the ghost hunters' medium claimed this.
Of course Benigna could have been the only one around when she poisoned the kids, the other staff might have been away (e.g. at a convention). She hadn't worked there for a long time, so she could've been the one who had to stay. But she had recently lost her own son, so would they leave her alone? And how could she explain the children having disappeared?