Twelvelegs, I think if you search threads on Mumsnet about dogs you'll find lots of Mumsnetters are happy to leave their children with even pit bulls, because of course their lovely little pet would never attack anyone, would they?
Open your eyes and look around you. You might not like that this happens, but it does. People nip out to the shop or to pick up other children, leaving younger kids unattended for a time. Usually because they don't have anyone to look after them for a couple of minutes.
I think it says more about this society that we no longer have that sense of community that we can ask someone to keep an eye on our kids. Most people are now so paranoid that paedos lurk round every corner they would rather leave their dc's unattended than have a neighbour round to keep an eye on them. The OP has offered her brother help, but many people don't. I remember starting a thread on her once asking if it would be ok to leave my 7yo dd alone for half an hour after school just whilst I got back from work. In the end I didn't, decided it was too risky. But prior to that I had actually stood outside the school gates asking other mothers if they would look after her for half an hour after school whilst I got back. None of them would. I bet they'd be on here judging me if I had left her though.
And saying the McCanns were ignorant and stupid is not very nice or helpful. When you're in that situation, where everyone is doing it, it's very tempting to fall into their way of thinking. The McCanns could see their apartment door from the restaurant, fwiw. Yes I don't think they should have done it and I wouldn't. But I can understand that you do stupid things when everyone else does. There was a psychological experiment done about this very thing, they persuaded one person to do something really stupid just because everyone else was doing it and it seemed normal for them to do it. It's so easy to fall into that. So rather than condemn, which is the easiest thing in the world to do, why not try to understand and perhaps offer a bit of help? There might be a mum in your neighbourhood who is struggling to find a bit of childcare and you might be able to help.