I dont think the OP is evil crap mum of the year for having her children run off briefly - despite what anyone says, I think this could happen to any of us or has done when they are a certain age, special needs or not. It doesn't sound like they went far and she was right behind them, so for that happening, realistically, she was not being unreasonable. Its how you deal with it that matters.
For ripping into the woman, I think she was a bit unreasonable, particularly in the way she went about it. It sounded like quite a confrontation. A short sharp dissaproving retort may have actually been more effective.
I also feel the woman in the shop was unreasonable for what she said, but not necessarily unreasonable for saying something.
In my situation, if it had been me, however, whilst I wouldn't speak to my children like that, or anyone else's, if a woman in a shop who I was never likely to see again spoke to my child like that, I'd let them take the earful, and I'd follow it up with "wow - you really upset that lady. Lets go and apologise". It keeps you the better person, the other person, if they have over-reacted, would probably feel like a prize heel and the child may well be reeling from the super strong telling off but that's not always a bad thing.
I think the only time I'd rip into someone in front of my child is if they issued a personal insult about my child, like perhaps said something horrible about their appearance or something - but even then I think I'd keep it short and sharp.
OP, I think you have been hammered a bit on this thread, some of it has been harsh, but then again, I do think you had decided you were not unreasonable and were not expecting people to think you were. I think there was an element of unreasonableness about how you went about responding to this woman but it is understandable. I can't bring myself to say YABU but I can't quite say you are not either. So I'll give you a YASONBUBOTOHYSOA which is, of course, you are sort of not being unreasonable but on the other had you sort of are! 