Glad I’m not forgetting things in my dotteringly old age!

The reasoning for why this is, as I remember it, is that most poisons found in flora are deterrents, “designed” to dissuade mammals from consuming whichever part is poisonous, and not outright deadly. That’s why many fruits are perfectly edible even when the rest of the plant is toxic (yew berries, for example); it’s beneficial for the plant to have it’s seeds spread, but eating the stems / leaves / roots are all extremely harmful to the plant. Children, with their much lower body mass and tendency to stick everything into their mouths as a first reaction, are much more likely to be killed by a small amount of a poisonous plant and as a result need to be far more sensitive to trace amounts of a toxic substance in order to survive. Sweet poisons tend to be manufactured rather than naturally occurring, so wouldn’t have had an impact on the evolution of this trait.