depending on the situation, I do!
if the child is in my care, then yes.
if the child is with me and also their mum is too, and mine were playing up as are theirs, and they all need telling off, then my friend and I share the yelling......I and she/they find that another mum doing the yelling produces better and more instant results.
I even picked up two boys in the playground once (they were wrestling and punching each other to the ground!!), and marched them into the school and handed them over to a teacher before now!!!
Maybe I am unusual, but I nannied before my own children, and also cared for family kiddies too, and I was always assumed to be in charge of discipline, and so I have carried it on into motherhood, but then again, I would back anyone up who felt that MY children needed to be told off.
To be perfectly honest, I get so tired of hearing my own voice telling mine off, that I truly love it if other people take over, plus it proves to my children that I am not alone in being riled by their behaviour, as sometimes I fear that they get deaf to my voice and think that I am too strict.
I think that if we all did take more responsibilty, and other parents let us tell other children off, then we would start having children give adults more respect again, and it wouldn't be so damn terrifying meeting crowds of teens on the streets......if they were being loud and we could tell them to shut up without the fear of being beaten by them and their parents, we would not now have the 'hoodie society' that we now have.
I don't think however that I tell other kiddies off over and above their parents.......luckily so far I, and my closest friends, all have similar approaches to how we wish our children to behave.....and I certainly expect mine to adhere to my friends house rules meaning I expect them to be told off occasionly.
I also make sure that if someone DOES tell my child/ren off, they listen and learn from it.