Who says it was Calpol? If you read the Portuguse detective's book he was convinced that they gave them drugs to sleep, as in drugs they had access to as Doctor's, and not just Calpol which is why they refused to let the Police have the children tested until 3 weeks later when they said that they could which is way to late to test for anything.
In fact, his theory is that they accidently overdosed her and she died and they disposed of the body and made up the story about the kidnapping.
And wasn't the restaurant where they were over 400 yards away? Hardly just a few metres away like you say? That's like me going to the village pub and leaving the children at home by themselves. That's enough for social services and the police to prosecute in this country i beleive.