If kids are with parents then they should be keeping them in check...if not its always better to intervene if you have to with kind words directed at the children rather than harsh words.
i agree, if youre gonna say anything to a child then say something nicely.
yes parents should be watching their children but with more than one when out in a group of friends it often happens when a parent will have words with a group of the kids rather than just single their own child out.
If mine were being naughty id not worry if my friends said anything to them, or me. what i would object to is anyone taking it further than that