Corbyn - the situation had been dealt with and the trolleys were back as they should be. If the OP had been standing at the boy's side when the other customer intervened, she'd have been in a position to say, "Oh, yes, he knows it's not nice and we've dealt with it now, everything ship-shape', big smile, end of story. But she was in the car having put the shopping away and driving to the trolley park to pick him up. (This is perfectly acceptable thing to do - the person who thinks that asking a 9 year old to do a task like return the trolley without close supervision, is borderline negligent and risking abduction, is a bit bonkers, in my view anyway) The boy is told off - again, for something his mother has already told him off for, and which he has put right - and he lacks the poise and self-assurance to say, "Yes, sorry, I know and I've put it right. My mum told me." He's flustered and embarrassed and he back-chats.
- Random woman probably shouldn't have had a go at the boy for something that was already put right, but people often like to have a moan, especially at easy targets.
- Boy shouldn't have backchatted her, but 9 year olds don't usually have the social skills to be polite and assertive rather than snotty and annoying. Especially when pulled up on something they think they've already put right.
-OP probably shouldn't have made such heavy weather of it, but on hearing that the boy had been 'cheeky', just briskly said, "Dear me, well that won't do: say sorry, DS, and jump in. Have a good day, byeee!' and then sorted out any fall-out with her son afterwards. "It's not fair - she told me off and I'd already put it right" "No, I suppose it's not strictly fair, but sometimes that's going to happen, so the best thing is not to muck about with the trolleys in the first place. You'd have been better off saying, I know, my mum already said. What shall we have for dinner?"
But sometimes people don't react exactly proportionately, do they? Bit like Mumsnet!