(2) After a year of this, the wife has a baby and works just 20 hrs a week, still at $15/hr. The husband has been promoted to store manager or higher (convenience and fast-food store chains are desperate for workers they can promote) and makes $25/hr.
(3) Their new combined income is $67,600--less than before, but still enough for a good life except in a few megalopolises. This is a completely realistic scenario, and not even demanding (the husband could easily continue to work 48 hrs a week). And it's been done with jobs at convenience stores.
(4) If you come up with places where the starting wages are lower, they're highly likely to be in poor states where the cost of living is lower. It's still a realistic scenario. Making enough money to support a family is easy in the United State if you're willing to work. Easier than it was in the fabled 1950s.
The American economy would shit itself if it were possible to simply be a quiet family that works retail jobs and raises a kid on 1 and a half incomes. Every aspect of American life for lower income people is presumed to have debt involved. Credit cards, home loans, medical debt, car payments. Those are how the gears turn and it's what the capitalist class wants. They don't want stable, financially independent workers. They want debt slaves who live off payday loans.
Maybe it is possible to live like Dr. Racist is suggesting, but it would involve a series of implausible circumstances to the point it's not worth considering. It would be a family who somehow has no debt, a stable living circumstance, an assumed promotion after one year, no medical emergencies, no accidents, no layoffs, no sudden natural disasters, nothing. It would be threading a needle every single day, for the rest of their lives. Getting a royal flush every day.