When my DS (age 9) played in his first Magic tournament, we were quite worried about the setup and how it would go being with all the teens and adults. My DH stayed (and played) to keep an eye on things. The only problem was an adult that lost to my DS and threw a tantrum with lots of curse words. My 9 year old said "Hello, 9 year old here- careful!" The guy immediately shut up and was very embarrassed by his behavior. From that lesson- I recommend a quick conversation with your kids about not only being gracious winners and losers, but what to do if someone else isn't, particularly if that person is an adult.
I would have preferred my son say nothing, so as to not create more conflict. But it turned out ok.
