If other parents are not present and you’re addressing a group of children, try to be as succinct and direct in your approach. Do not make any personal attacks or single out any of the children.
For example, if children are handling a valuable object carelessly, a simple, "Children, please put down that vase now," will suffice. Be as firm as you can, stand your ground and offer suggestions for other more appropriate activities the children should do instead of playing with fragile items.