Just because the consequences of that evening led to something far worse that also negatively impacted them doesn't imply that they should not face the legal consequences of their actions, it would only mitigate the sentence, not remove the charge to answer...whether that is simply a recognition by the authorities that they did deserve censure or that, like the Edwards Case, deserve something more is not really the point...the point is that they, as anyone else, should be held accountable for their indifference to the welfare of their Children...not only Madeline, but also the twin babies, in fact more so the Babies.
If we follow your logic then in any case where a case of negligence is suspected, the consequences of that neglect whether direct or indirect would determine whether the person(s) are to be held accountable (investigated and prosecuted) for the initial negligence...I do not think that can be justified in a fair objective legal system.