dullard,
While I agree with your description of how a 2X increase in labor costs will not cause Happy Meals to cost twice as much, that still isn't enough information to say, "Therefore, increasing the minimum wage will be a Good Thing."
First of all, my perception is that the percentage of people making minimum wage is fairly small. That could be incorrect -- but that's just my perception. The Wendy's near me is hiring at $8 to start! So, it's even unclear how much raising minimum wage would affect the economy.
But let's say, for example, that everyone that works at McDonalds makes minimum wage. Using your example, if labor costs double, then the employees of McDonalds are making more money, and the cost of a Happy Meal increases by 50 cents. However, the salaries of those people who do NOT work at McDonalds does NOT increase. They now have the same amount of money to spend on more costly goods. As a result, those employees at McDonalds (and the other small percentage of people making minimum wage) are making more money, but the rest of the country is WORSE off. Unless 50%+ of the people in the country are currently making minimum wage, the economy as a whole will be hurt.
Finally, something should be said about demand curves (everyone who doesn't give a lick about economics, stop reading here). If the elasticity of demand for Happy Meals is high (that is, the demand curve slopes downward steeply), that extra 50 cents (~ 25% of the original cost) will cause a large drop in demand for Happy Meals. Given that fast food has many good substitutes, I think a high elasticity is a fair assumption. That being the case, McDonalds will likely lose money in the long run, unless the people making minimum wage now spend a huge amount of money at McDonalds.