As a father of a 7 year old who play's and loves the game (the reason I started was to help him learn and now we do it together) I feel it comes down to what the parents do. At the last pre-release my son did not get the best cards for the build a deck and ended up loosing all the games and he was bumed (no tantrum though, but he did want to go home-nothing a bit of good traing couldn't cure though) but it was a good learning experience for him. It helped that I was there and that I also lost most of my games-we had something in common to talk about. In short, the parents play a large role-there is a lot they can do to help a kid through these types of problems. If the same kid is continually having problems the parents should chatted with constructively. 7 is still quite young (1st or 2nd grade) and are still developing a lot of their people skills. If it does get too bad though and is perhaps ruining it for others in his age group it may be time for a time out - and it needs to be enforced or the problem may just get worse. Empty punishments have always ended badly for me as a parent-the kids learn the wrong lesson.
As to loosing - I hate it. I throw a tantrum.... Jokes, but I do hate it - I play to win. Because I play to win I try to learn from the loss-what could I have done better - no deck is unbeatable. I am always reworking/trying new things with mine (though most of the time I am playing a 7 year old...who does beat me-I think his current deck is better than mine). Play to win-and the playing is not just 1 game it is before the game starts, it is going over your cards, practicing, changing, winning, lossing, and doing it all again and again and then winning, winning, winning.