If i were to see children left in the car I would wait for 5-10 mins, if the parents hadn't returned by then I would call the police. In the situation of the 8 year old, I may not, and can say that like the op i would have probably made the same choice.
I have left ds in the car to buy petrol but he is never out of my line of sight. If I have to go into a supermarket he comes in with me, even if it is just for one thing. There are too many things that can go wrong in even a very short period of time.
ETA that I think the last story of the mum with three kids being helped in the supermarket is a lovely story. I wish more people were like this, when my ds has lost it in the shops I have been known to abandon my trolley and leave in tears. In moment like these I wish that I had encountered lovely people like her rather than the death stares I have so often gotten
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