In Iceland the parents will park a baby carriage outside with the baby in it, then go into a store and go shopping, leaving the baby outside. They might do that in Denmark too, but I saw it in Iceland. Doing the same in the USA would probably get you arrested.
As far as the McCann case from Portugal is concerned, it's not customary to do that in UK (parents were British) but the parents were at a restaurant within yards of the rooms and had some system where somebody from their group went to check on everyone's kids every half hour. I'm sure they thought it would be fine. The person who did the half hour check before Madeleine disappeared didn't actually go into the bedroom and didn't check to see if she was in bed. Just went in to see if everything was quiet. Her mother went in on the next half hour and found she was missing.
It goes to show how quickly an abduction could happen, since it's now widely accepted that Madeleine was abducted by someone from outside the family or the group of friends they were with. But they still park babies outside stores in Iceland.