When you don’t see the misbehavior

How should I handle situations where I didn't see who, for example, peed on the floor, or broke a toy, or took somethingthat wasn't theirs? Or when one of my children says her brother hit her when I didn't see it happen?

Also, you say to wait till the child is calm to do a correction , so when the misbehavior happens do I let the child know I'll be doing correction when they're calm or do I just wait till they're calm and pull them away from what they're doing and then do a correction?