I'm cool with strangers telling my kid to stop it, or be quiet.
My son is 2, and in the dreaded terrible 2s, and often times, what you say to them goes in one ear and out the other.
Every now and then I wish a stranger would tell my kid to quit acting up, or tell him to be quiet, because I know, if someone else said it to him, he'd likely be quiet, because, quite frankly, he doesn't care if his screaming bugs me, I'm his mom.
My point is...I would find it great if a stranger told my kid to be quiet, because that lets him know that its not just me that gets annoyed by his behavior.
I don't understand why most people get all up in arms about a stranger telling their kid to be quiet.