The economy is not an iron mine. New disposable income does not come from some finite source. An economy is money moving in circulation. People stop spending as much, you get a recession. People spending more, economic boom. More money going to the working class, which spends 100% of their available funds, means more money out, more money flow and a better economy.
If that was true, hyperinflation would be the best thing for an economy, because hyperinflation causes a tremendous increase in "money flow" as people desperately try to spend their rapidly depreciating currency.
The truth is, disposable income does come from a finite source, because the supply of goods available in an economy is finite. The minimum wage is a price control, and like all price controls, it distorts the economy. A price ceiling causes a shortage, and a price floor causes a surplus. The minimum wage is a price floor, which means it causes a surplus. What does it cause a surplus of? It causes a surplus of labor, which is also known as "unemployment".
You may respond, "Can't employers simply raise their prices so that no one has to lose their job?" The problem is that employers have already set the price of the goods that they are selling to the level that clears the market. In other words, if the employer raises his prices, he will lose customers (if this weren't true, then he would have increased his prices long ago). In summary, to the extent that employers raise their prices, they are losing customers, which ultimately means less work for their employees, which ultimately increases unemployment among marginal workers (e.g. teenagers).