The McCann parents did not make a mistake. What they did was deliberate. They were the ones who left the children alone. Parents are responsible for their children 24/7. If the McCann parents were so hungry and thirsty that they had to eat and drink at that time in the evening then they should have gone out one at a time.
Of course, we dont live in a perfect world and that is why they should have thought about the situation before they turned the key in the lock. As the questioner mentioned, anything could have happened to those children without any outside interference.
I was just about to type 'what were those parents thinking' and then I realised what I should be typing is WHY DIDNT THOSE PARENTS THINK BEFORE THEY ACTED?
As in all these situations it is the child who will pay the ultimate price whether she is found alive or not.
I am a parent and a grandparent and I know what it is like to care for children. Until I decided that my children were old enough and well equipped to cope alone I never left them alone. When you have children their well-being is your only priority. What you yourself may want to do only appears on the 'to do' list when your childrens' needs have been met.
No, I am not perfect, I make mistakes. This situation was not a mistake on their part.
My thoughts are with Maddie and I hope that she is safe.