Lack of demand, on the other hand...
Yes capitalism is amoral so people will certainly be willing to pay to increase salary, but they are also willing to buy machines, outsource work, make customers do more and other things that reduce the need for employees.
Where do you think the money for EITC/WTC comes from?
The catch to this line of reasoning is that you might have a perfectly well-paying job when the kid is born, but several years later endure a drastic shift in your professional life that's out of your control.
So I don't know. "Must pay enough to support family of 4" is incredibly arbitrary, and EITCs for low-income families (as another poster mentioned) sounds icky to me, since now everyone else is just subsidizing low-wage employers. Not sure what a good solution might look like.