As a student of the toxicology literature, I am happy to inform you there are plenty of data. Indeed, one of the advocacy groups, Environmental Working Group, in one of their recent reports cites quite a bit of it. It turns out that the so-called neurodevelopmental toxicity only seems to happen when neonate rats have been exposed to doses at the maximum tolerated level. Just because a rat cannot tell you it's feeling nauseous today doesn't mean that rat is not one sick puppy. In other words, those maximum tolerated doses routinely used in mechanistic studies are astonishingly high compared to environmental levels of residues.
Biochemical reactions do have thresholds according to kinetic theory.
For that reason, the dose does make the poison, regardless of the effect measured. But some might say that we have to look at the worst case. Well, for OPs we actually have human testing data, and human cholinesterase seems to be the most sensitive biomarker of probable effects. Cholinesterase sensitivity is a key predictive parameter, because the neurodevelopmental effects seen in rat neonates are accompanied by significant cholinesterase depression. Indeed, it is excessive acetylcholine at the synapse which seems to be a factor affecting neurodevelopment.