I think the only children you can change are your own. However i was in a position not to long ago when my daughters friends(sisters) were over with their Mother for a playdate the sisters wanted to jump on the couch. My daughter actually told them that this was not allowed, I waited for the mother to say something, she didnt , they wanted to jump more, then i decided to step in, and let them know the rules of my home. What are your rules for disciplining other children when their parents are around? Does it depend how comfortable you are with parents ?