I think that if we took out of the equation that other people are annoyed at the noise our child is making, we would have more headspace to deal with our children more constructively in public. That's what is a huge factor in frustration - trying to get our kids to squeeze into the mould that other people want them in...i.e. quietly following you and making as minimal a presence as possible.
I think it DOES help a parent a lot to make eye contact, maybe say something that implies you can relate. Because then that parent feels more normal, and not as if they are being judged for letting their kids 'get out of hand'. I think it has a lot more to do (the frustration that is) with how that parent perceives other people judging their parenting, than the behaviour of the child, in many cases.
We're not in a very child-friendly society and this adds to the frustration. People tend to ignore you in that situation and you just know it's because they are not dealing well with the children. If only they knew that it would make life easier for EVERYONE if they could just look at you and say 'just one of those days, huh?' - I've said that before to mothers with their hands full, and I have seen the weight lift off their shoulders! I'm telling them that it's ok by me if the kids are reacting adversely to an environment that is very geared towards adults and is rubbing them the wrong way. It's neither of their fault, in most cases.
A bit of solidarity really helps when out and about!! I'd hate to be left out to dry by other parents when Oscar is of tantrum age, and I've been acknowledging parents 'in need' for a long time, having seen these patterns for years working in the retail trade.
I must admit that one time I visibly flinched when a child just randomly squealed very loudly and it pierced through my head! The mother looked offended and I just told her that the noise went through my head, but I didn't ignore the child because of it. If someone bumps you, you stumble. My hearing was assaulted with a very sharp noise and I reacted.
I have a colleague who is very critical of screaming kids in shopping centres. She has no kids! She says it's the parents' fault and they should control the kids. This kind of person contributes to a child-unfriendly society. I really resent it. My uni has a policy saying that children are allowed in study areas like computer labs and libraries so long as they are virtually undetectable (not in those words, but it's trying to say that)...
Maybe it helps me, when I'm in a situation with a frazzled mummy and kids in a shopping centre, to know that it's an adult environment and the kids are overloaded with all the messages that are screaming from every shop, every department. It helps me understand why her kids are 'ratty' and to let her know that she is not responsible for my peace and quiet.
Then again, I really do avoid shopping centres like the plague, even without Oscar!