This is quite an interesting subject.
How would you talk to another child who is doing something perhaps they shouldn't?
i.e.
a)about to run across a road
b)hitting your child
c)hitting another child who is not your own but you can see
d)playing roughly with other children
I think I wouldn't necessarily shout at them (unless they were much older/bigger etc) but perhaps say "don't do that please...you could hurt [insert childs name]" or "oh dear be careful' or 'DONT HIT!'
Then again, who knows, if you see your child getting hurt it's like a red rag to a bull!