People really just weren't ready for social media or having miniature computer-cameras at their disposal constantly.
If your child is having a meltdown of throwing a tantrum then you as a parent have a job to be doing. Put the phone back in your pocket or your purse and attend to your kid. Figure out what the problem is, do what you need to do to calm them down and consider that maybe if this is a frequent occurrence that perhaps you as a parent fucked up somewhere and you're going to have to be responsible by working to correct whatever mistakes you made that brought this bad habit into existence.
I'm sorry that happened to you but I appreciate you warning people of the dangers of being confrontational with these people.
It's not worth it. You won't change their mind, people nearby aren't going to clap for you, you're just unnecessarily putting yourself in harms way because many of these people are not kind, they will respond aggressively.