I have just looked on google to see if I could come across anything as to why the McCann's were not prosecuted for leaving their three children alone that night. 

I came across this whereby the Portugese thought it was a 'Peculiar Custom of people in Britain to do that'

MADELEINE McCann’s parents were not prosecuted for abandoning their daughter
while they dined near by because of “compassion”, law chiefs have claimed.

Portuguese officials thought Brits had “peculiar customs” where it was “natural
for them to leave the two-year-old twin siblings and the other
three-year-old child alone”.

According
to the Daily Star, Mr Pereira said: “The error was not constituting the
parents as arguidos for the crime of abandonment.

“At the beginning there was an extraordinary and ridiculous theory that
said the English have very peculiar cultural customs.

“And therefore it was natural for them to leave the two-year-old twin siblings
and the other three-year-old child alone in a bedroom for the parents to go
out a few hundred metres away to socialise with their friends.’’