Ohh my.. This story makes me well up!!.
Your boys must have felt so sad and probably quite intimidated and scared too, especially the for the Autistic brother.
We have a 4ry old boy who is currently being diagnosed to fit somewhere on the Autism spectrum so I can relate.
I feel quite anxious for him when he gets older and how he will have to deal with these situations on his own, this story further worries me.
The world is hard and full of mistakes and this experience must be massively difficult for an Autistic sufferer to understand.
I hope they do learn from this and realise it was a big mistake.
Maybe take them both out to the shops again (probs not Asda) and repeat the process to show them it works 99.9% of the time.? Try to show them how the mistake happened? (if you can find any reason)
I’d be asking the manager to arrange some sort of complimentary behind the scenes visit to the store. It could be quite interesting and also help to restore your boy’s trust in shopping.