Just to add to this, I have also come across references to "ptomaine poisoning". After some research, I discovered that it was thought that ptomaines, found in decaying animal and vegetable matter, had a serious and sometimes fatal effect on the body when ingested. We now know that food poisoning is due to the action of bacteria such as salmonella and e-coli. But again, the newspapers scandalised the event as if it had been deliberate. This was later, 1903.
The Inquest found that there was no poison in the house but, as has been mentioned, there was lead paint, arsenic, etc. The girl had apparently always been a sickly child so it's possible there was a serious health condition. Or she could have eaten something on her way to school.