We did this last week - we locked DS2 (who is 8) in the car because DH thought he had gone into a hotel with me and I thought he would stick with DH because he was faffing around getting something out of the car. It took a few minutes before we realised we hadn't got him with us. I went and got him from the car. No harm done although he was beginning to get upset, mainly because the car was just about to go nuts - the alarm was set and it would have gone off if we had left him a few moments longer.
8 yr olds aren't babies. We don't tie them to us every second of every day and they can wander off a bit - they probably should in fact, learn a bit of independence. It isn't surprising this happened.
I personally think this is more a reflection on the security than the PM who made an honest mistake that any parent could make. The protection officers are paid not to make this sort of mistake however.