I don’t think it’s that hard to fathom.
For a start, you’d assume a parent is in there with the child. You might assume the child tends to be a ‘crier’, or is being difficult about bed times. You might assume you’d get retaliation from the parent if you intervene.
Also I imagine being a permanent resident in the middle of a holiday complex like that, you tended to hear all sorts, people coming and going from bars late at night etc. Maybe this woman had previous experience of calling reception about disturbances only to be told it wasn’t their problem etc.
What’s much harder to fathom is why any parent would leave their children knowing that two of them had been crying the previous night 
And of course I fully expect the usual posters will bang on now that we’re not allowed to say that, or that it was quite common to leave your kids while going out for the evening in 2007... but frankly I find that The unfathomable thing - that they knew the kids had been crying the previous night.
Some of the things in the media are horrific; it’s hard to know how much is based on actual fact from the police forces and how much is the media trying to make links and keep the story out there... but it’s awful to think of what was going on.
If there is video evidence that MM is dead then it seems impossible there’s anything that links CB to those videos for certain, because presumably they’d have brought charges. He’s been under scrutiny for years. If we’re talking about a paedophile ring then there would have been various people coming and going from these various houses and other buildings too, so even identifying a location is circumstantial rather than proof. I really hope whoever is responsible for any of these appalling crimes is brought to justice though I fear that there just may never be evidence that can stand up in court. Combination of at least one attacker being forensically aware along with totally inept police investigations means it’s a hard task to find justice now. 