Well isn't hindsight a wonderful thing
The parents took a calculated risk - they are not stupid people and in their line of work will be well used to evaluating risk and determining what is acceptable risk or not.
The fact of the matter is that the risk of abduction in such a situation is infintesimally small. So small it is probably almost not measureable. Way less than the risk you take when you put your baby in a car, way less than when you decide to cross teh road with your child in a buggy, way less than even putting your child safely in their cot to sleep.
This couple will have discussed and evaluated the risk and put in place additional safeguards such as returnign to the room regularly to check the children. They were very very unlucky. In this case they were the random statistic - the outlier - the one that is the exception to the rule. With heartbreaking consequences
The fact that you might have evaluated the risk differently to them, is neither here nor there. We all have different risk profiles - of what is acceptable risk or not. And of course one might consider in hindsight that the couple misread the risks involved (although I personally think not).
What is lamentable, is the way that many people are using their own interpretation of risk to condemn what the family decided to do. This is judging at its worst imv. Totally inappropriate, not least in its timing!