Firstly, who the hell keeps their kids sleeping patterns on holiday? Most parents would let their kids stay with them in a pushchair and let them fall asleep with them by their sides.
Secondly, i can't believe anyone thinks it's ok to leave their kids alone, in an unlocked apartment with a road on one side and a swimming pool 75 metres between them and the apartment.
When they're sat selfishly enjoying themselves having tapas and wine, what was to stop Madeline walking out either into the road or falling into the pool? They probably wouldn't hear either happen from where they were.
Cuntish parenting.