There is no excuse for leaving your children (it wasn't just Madelaine, but her 2-year- old-twin siblings as well) unattended in an unlocked apartment while you go out for dinner for one and a half hours. There is not only the risk of adbuction, but also what would happen if there was a fire? The whole coming-and-checking-on-them-every-20-mins things is not a safeguard at all. If I were a child abductor observing a family from which I wanted to adbuct a child and I saw that pattern of behaviour, it would be very easy to time when to go into the apartment. Kate McCann also admitted in that book (there were exerpts in one of the papers) that this wasn't the first time on the holiday that they had left the kids unattended while going out in the evening. Again, if a child adbuctor was observing this (which is likely, no one wanders by and abducts children without planning it out) it would establish a pattern of behaviour that was easy to exploit.
It is every parent's responsibility, and should be their instinct, to protect their children every minute of every day, whether that be supervising them yourself, putting them in the care of a responsible other, or at the very least locking the doors and windows. I am a parent and I would be disgusted with myself and would want no pity if I were to show such neglect. I feel pity for the child, but not the parents.