That is part of the problem. If it weren't for using a cutoff, then it would be the case that "proving" "! forall X P(X)" with high confidence would be evidence for "for many X, !P(X)", as several of the comments below are claiming.

But even if they'd used some kind of Bayesian approach, assuming that all children are identical would still mean they were measuring evidence about the claim "X affects all Y", and that evidence could not be used to conclusively refute the claim that X affects some fraction of Y.

Using a cutoff, though, isn't an error. It's a non-Bayesian statistical approach that loses a lot of information, but it can give useful answers. It would be difficult to use a Bayesian approach in any food toxicity study, because setting the priors would be a political problem. They did their statistical analysis correctly.