I don't glare at parents when a child is being bratty and unruly in a grocery (or other) store - usually the look on their face tells me they are at their wits end and their child isn't listening and they really DO just want to get out in one piece.

I give the child a stare that would chill steam. On many MANY occasions, the child, who might be used to manipulating a parent, won't try the same to a strange adult. I've had lots of them cut the crap IMMEDIATELY. Yeah, they're probably scared. Or at least surprised.

When the child stops screaming (or whatever he/she was doing) I then smile. Doubtful they are traumatized for life.