We often find ourselves dining out with a family member who is embarrassingly rude to the waitstaff. Consequently, we leave a larger than normal tip when this happens. I also try to give a sympathetic nod or say something gracious to the server, whenever it is possible without exacerbating things.
If you are in a similar situation (a badly-behaved child, an unusual mess, etc.), do you leave a sympathy tip? If so, what percentage above your normal tip? Not that you can ever compensate for unkindness, but since money is the only way I can make up for this family member's behavior, that is the means I use.