You missed out a rather important part of the question here because you are relying purely on subjectivism. The question should be reformed as:
Did they probably feel it was perfectly safe, and ought they to have felt it was perfectly safe?
It is less certain to say yes to this given the facts of the case (~70m away, unlocked door, obscured view- whether by darkness, foliage, the plastic window- no sight whatsoever of several entry points to the property, chose not to use the night creche) and that is why most of the debate in this thread has been generated by the fundamental point of the standard of fiduciary duty that a parent should have for their child.
Ultimately, jail time for neglect doesn't help the two remaining children (as I highly doubt they'd leave their children in such circumstances again), but to suggest, as they have inferred in several interviews that these actions are perfectly reasonable and constitute the expected standard of fiduciary duty is, for many, a claim too far.
Very good point.
My main argument on the situation they found themselves in was where they were.
I obviously cannot know this but I’d guess that they wouldn’t have done the same in a major city or even a built up town, if they were away for a city break.
I think because of how relaxed, quiet, family focused and because of their being zero crime in the resort - they felt more comfortable doing something a parent wouldn’t necessarily do elsehwere.
They ought to not have felt safe enough as they paid the ultimate price but the question on whether it was reasonable or not can only be answered by people who have been in a similar circumstance or were there at the time.
What I mean by this is that being in the situation yourself allows to gauge the atmosphere in the resort/area and the type of people you come across on a daily basis.
I said in my opening post on this thread they didn’t do anything wrong, I do take that back somewhat, they obviously could have done more, I just don’t believe their mistakes are guilty of gross negligence.