Yelling at other's kids

So would you ever discipline/yell at/correct another person's kids? So not too long ago we were at a cider mill that has this big sand box type thing only it's filled w corn instead of sand. Kind of like a huge sensory box type thing that kids can sit in, lay in, play in. Lots of kids were in there and loving it, Having a great time. One kid was throwing corn at random kids and nobody was saying anything to him. My sister finally went up to the boy and told him please don't throw the corn, someone can get hurt. Then the mom immediately approached from a couple feet away and stepped in and told my sister "sorry" and talked to her boy. He was probably like maybe 4 years old. Would you do the same w a strangers kid? Or should it be up to the parent only to correct/disciple him?

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