Back in the day at British holiday camps, parents would get their children off to sleep and head off to the entertainment centre for a drink. Staff would patrol the chalets and if they heard children crying inside, the chalet number would come up on a display in the entertainment hall, to alert the parents.
The McCanns apartment was within sight of the bar they were eating/drinking at. My guess is that the abducter knew the McCanns routine and took their opportunity. I found the treatment that they received after Maddies disappearance, very unsavory. I can't imagine the stress that they must have been under.