In Flowers in the Attic , the children were poisoned with small, repeated doses of arsenic mixed into their food, especially the cookies. The poisoning is ultimately revealed to have been done by their mother, Corrine, as part of her plan to protect her inheritance.

What that meant

The poison was not a one-time event; it was fed to them over time so it looked like illness rather than obvious poisoning.

Why it mattered

That detail is one of the story’s biggest shocks, because the children initially suspect their grandmother before learning the truth about their mother.

TL;DR: the children were poisoned with arsenic, and Corrine was the one doing it.