Being autistic myself, and also being a mother to children on the Autism Spectrum I can say that I've been in both situations quite a few times and I don't think there is a right or a wrong answer. Like many other situations in life, the choice to stand up and speak out is individual.
It's really hard for me to be in public and see how common it is for people to react negatively to others, no matter what the issue is. I think perhaps because I'm actively not looking at them, I am picking up on everything else. I can feel and hear far more than most neurotypical people around me can. I don't see it though, I feel it. This type of emotional response is more common than I once thought, common among those of us with an Autism Spectrum Disorder anyway. Once thought to lack empathy, professionals are starting to realize that our emotional responses are often too much, not too little. And often times the emotions are being received at such a fast rate, that we become overwhelmed. Bombarded and assaulted by our senses. As always, research is ongoing as to why this occurs.
I usually know instantly when I'm around someone that is cognitively or neurologically different somehow. There is an unspoken language I think. I see those subtle signs of stress, or agitation. I can see that they aren't gazing at a flower, or walking by daydreaming. Instead I can see that they're counting the petals on the flower, and humming the theme song to a TV show over and over while running their index finger round and round on their thumb nail.
I see these things because I live with them daily in myself. (this is of course a doubled edged sword. It means I am capable of seeing when my autistic children are coming close to meltdown point, but sometimes my mommy-mode mind isn't listening. This is incredibly frustrating for me and I will tell you honestly that I do feel guilty every day for trying too hard to be a neurotypical mom when I'm not. It's a difficult line to ride.)
I can't tell you the number of times I've seen children flapping, spinning, or scripting to themselves and watched interactions with their parents and been shocked to learn later (in cases that they become acquaintances )that the parents don't see their children in distress at all. In fact, they complain of all the typical autism symptoms, but don't realize their child is autistic at all. Meltdowns are called temper tantrums, stimming is called "being a freak" or the child " being a weirdo." and other behaviors are receive responses like, "He is all boy!" and "she's just a sensitive girl that's shy and keeps to herself."
Now I'm all for not labeling everyone just for the heck of it, but I still cannot fathom in this day and age how people can't know their child is autistic. Then I remember...before autism was a part of our lives; I didn't know either. I just didn't know.
It's like not knowing anything about Parry-Romberg syndrome or Ribose-5-phosphate isomerase deficiency. Why don't I know anything about those disorders? Simple. Its because they haven't impacted my life yet. Is that a great way to live, running around completely ignorant that these disorders are affecting children? No. Its also not feasible that I learn about every disorder and disease out there. (at least not without obtaining a degree while doing so) Familiarizing myself with the "most common" diseases and syndromes is do-able. And Autism should be on that list for people I think. I think its do-able for every person out there. (Of course I also believe everyone should know how to perform CPR.)
It takes time though. Awareness doesn't happen overnight. To be honest, the arguing among professionals about the different types of autism and where and how they're going to change things is off-putting to people. No one wants to hear more about a subject that is so filled with emotion and anger. Its like shying away from discussing politics. Autism has become like that. A subject that seems to have only two sides, those that know nothing and those that think they know everything.
Finding common ground and coming together to help people become more aware needs to happen first within the autism community, before we can reach others. Its just surrounded by controversy and drama. How can I even begin to talk to people about it when many have their own opinions about it already.
"Just another crackpot diagnosis."
"Just another excuse for poor behavior and bad parents."
"Yep sure, everyone has that nowadays."
Add in the vaccine debates and its just not friendly waters.
That being said, whether I speak up or not in a situation like the one above would depend on a number of factors. What exactly did they say? Was it loud enough for a lot of other people and/or the mother and child to hear? Was it very rude or very ignorant? How am I feeling that day? Have I met my social limit already? Am I already stimming like crazy myself and looking for the exit? Do I have my hands full with my own screaming kids? What do the people look like? Despite what they might have said, do they otherwise seem like they might accept me saying something to them, or do they look like they might scream at me for butting my nose in? Do I even have enough reserve left for me to really look at them and not just look around them? If I don't have enough reserve left to even look at them to assess the situation, then I won't attempt to say anything at all.
I try really hard to look at mothers in those situations and give them my "I know how hard it is, I understand" look. (which may come across as the creepy I'm watching you look.. I might need to practice more) If I have enough reserve to do all those things above, and find them to seem to be people that would be somewhat kind if I spoke up.. then I might say something like, "She seemed to me like she might be ____(insert something here) developmentally delayed, autistic, etc. Sometimes depending on initial reaction, I might then say that my daughter is autistic as well and that many children with autism flap their hands (for example)
Stopping to gauge their reaction is important because then I try very hard to know if they want or need more information, or if they think I'm a weirdo and they're secretly wishing I'd fall off a cliff. Sometimes I botch this part up. When I get nervous and think I'm doing something socially inappropriate (butting my nose in) I often do it more, or continually, because I get stuck in a loop and panic. Not very helpful.
I tend to use my own experiences rather than the child/parent in question. That way I feel like I'm not broadcasting their business everywhere. And of course, I also preface anything I say with words that make it very clear that I don't know 100% that the child is autistic. She could have a brain tumor, or some other neurological disorder. I don't know.
However I do know that being rude isn't helpful to the mother or the child, and if I can make them think about the situation in a different light, then awareness has been achieved. Because in the end its not just about autism awareness. Its about acceptance, compassion and empathy for fellow humans. No matter what the diagnosis of the child might be, its secondary to the lesson in being kind and non judgmental towards other human beings.