My mum has Alzheimer's and we used to have merry hell taking her out in the year before she became so ill she had to go into a care home. She'd attempt to do exactly what that woman did OP. Because she simply can't stand piercing noises. She could cope with the low rumble adults make but young children would just 'go through her' - it's the pitch. So we'd try to find more 'adult' pub type places and, of course, we'd have a family near us with young kids (she was like a blooming magnet). It's not always that easy to move especially if it's an occasion like Mother's Day etc.,
But, I said she'd 'attempt' to behave like that woman. We'd know what our mum was about to do. We'd see the agitation start and we'd hear the 'oh for pity's sake shut that kid up' mumble. That would be our cue to go into distraction mode. Visit to the loo, little walk outside if it was a nice night. Flip out the phone and show her the latest Facebook messages from family. Agree to skip dessert and get something from the supermarket on the way home and get the bill pronto! And she'd always be told that we were exactly the same at that age so calm down. It didn't always work but we'd try. And this was a woman who adored kids, worked as a school crossing patrol person for 30 years and would have kids rush up to her for a hug when they went to cross the road with their parents - she just changed...
It's a shame you had to have this OP but I'd say a couple of things.
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People aren't necessarily picking on an autistic child. Sometimes they have hidden issues themselves which means two worlds have just collided. Don't take it to heart.
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Don't be put off taking your child out. We weren't put off taking mum out and it gave us great memories of her final 'out in the world' year before we had to find a safer place for her. Yes we sometimes had to quickly deal with a situation - she once had a poo accident in the middle of a cafe (that was fun...) but you learn. The only way your DS will learn what to do and you'll learn how to handle things, is for you to put yourself in those positions.
My friend, whose son is autistic, reckons McDs and places like Beefeaters or Brewers Fayres are her friend. McDs for a quick bite and to practice going out. The service is fast. Yes it may not be the best food (personally I love a McDs but that's another story) but because it's fast you can sit, eat and go. You can test the time limits as your DS grows. Brewers Fayres etc are great if they have a kids 'pit' that you can take DS into. Ask for a seat close to it so you can watch him as he gets older without having to constantly be by his side (unless he needs that support of course).
My DS loved Brewer's Fayre when he was younger - he could let off steam, shout, run about and his food would then miraculously appear. I think we knew the menu off by heart! And he's not autistic. We didn't eat out at posh places with him because boredom would set in. If it didn't have a kids menu and something for a kid to do we didn't open the door when he was under 5!
My friend used story boards to prepare her son when he was younger. They practised at home, sitting at the table quietly and pretending to order food just to prepare him for what to expect. That worked for him but every child, autistic or not, is different. It's only practice that will help you figure out what works. All I will say is that my friend's DS now as he enters his teens, with a plan in place each time, is a joy to take out. Loud, excitable, but a total joy and controllable but that's his level - everyone is different with different triggers, autistic or not.
I hope you have a happier time next time. I'm surprised the people the woman was with didn't say something to her (assuming someone was with her) because we'd have said something gently to her if it were our mum. Even though she couldn't help it any more than your DS could.