Paris Green was used to kill Roach's, ants and other insects. It was also used in Wallpaper and was latter discovered to cause many deaths due to inhalation. If Mrs. Purdy used Paris Green to treat insects and inhaled it she would have been effected. This may have been the original cause of her problem that lead up to the Murder/Suicide attempt.
In 'The Dead Files''Erie in Erieville' it is implicated that the children died, thus children Ghosts. The children did not die! The husband did not die! The only person who died was the wife/mother. This would indicate that she had been exposed to the poison for a longer time then anyone else, only proving the point that she had been breathing in the poison and was ill from it for quite a while before her acts.
For her Death to be used as a way of making fame for the Producer, the Medium and the Investigator is sad... especially since they did not present all the facts. Nor leave the watcher with a correct conclusion.
You do not have Child Ghosts when they have grown and died as adults, as a rule. And there may be graves but they were not of the children, as they did not die.