Don't take games / sport with the kids too seriously. Your purpose in playing with them should be to have fun, or, depending on the game, to help them be educated. If you take the whole thing seriously, what are you teaching them? That they can't have any fun? That life is all about winning and not about relationships and interactions? What sort of useful lessons do they learn if you become grouchy because you're losing? Winning is great - in the right circumstances, but games with the kids should not focus purely on winning.