"The waiter, who has asked not to be identified, said the nightly routine was "set in stone", making it easy for a potential abductor to "choose the right moment to take the child".
He said the group made a "special arrangement" to book the same table at 8pm every night so they could sit outside, 50 metres from where their children lay sleeping.
He told how the arrangement was widely known about by workers at the restaurant, who thought it was strange that other holiday-makers had to queue for reservations at the popular tapas bar.
But he said he only realised after Madeleine's disappearance that the routine could have made all eight of the group's children vulnerable to a potential abductor.
He then added "The apartments are all quite close to the pool, but there are trees in between so you can't see them from the restaurant.
"Even though they were checking their children every 20 minutes or so there was still a lot of time they were left alone when someone could have gone in and taken them."
"When you think back now, because the routine was so set in stone every night, if somebody had been watching the group they would definitely have been able to work out what was going on and choose the right moment to take the child."
His eyewitness account of the group's behaviour is the first time anyone from inside the tapas bar has spoken publicly about the events of May 3.
Kate and Gerry 'left Madeleine alone for three hours for four nights in a row' | Daily Mail Online
Tapas Book with names of group and their flat numbers
P.J. POLICE FILES: TAPAS BOOKINGS
The article said at 8pm but it is 8.30 pm in the files / Tapas service book