I think, as parents, we need to step up and first of all
make sure we set the example for them to follow. There was a parent at the state tournament that was standing behind his
opponent's coaches, screaming and cussing at them, the officials, the table workers. His son won so I can't imagine
his actions if he had lost. What kind of a role model does
this set for his son and other kids watching the episode?

Then it is, ultimately, the parent's responsibility to see
that their child behaves appropriately. Forbid them from attending the next tournament until they can handle their emotions, win or lose.