"Middle class families only have two children because they can only afford to send two to college. They have no desire to be put in the position of performing socioeconomic triage on their own children."
Right you are.
And when most people have only two children, a nation is in a demographic death spiral.
So, how can the middle classes be encouraged to have more children?
Assuming that college expense drives this in the US, three ways:
(1) Make college cheaper
(2) Allow people to keep more of their money, so that they could afford to send a third child to college.
(3) Make a college degree less important for hiring and high income jobs.
(1) can only be accomplished by the state shouldering some of the burden of college costs, since places like Harvard are not going to cut expenses, and landlords are not going to cut rents for college students.
(2) can only be accomplished by DRAMATICALLY cutting taxes, but that would entail a dramatic cut in what government does which may not be democratically sustainable.
(3) would require employers and holders of capital, who themselves are probably holders of degrees and advanced degrees, to hire outside of their own academic milieu, and trust their own abilities to train people without college educations to do what they want them to do.
(3) is by far the cleverest.
The wages of non-college educated workers relative to college educated workers have plummetted during the period in which (surprise!) middle class birth rates dropped. The glut of college graduates in the 60s reduced the value of a liberal arts degree to what had been that of a high school diploma a year before.
Another baleful result of such degree inflation is saddling young people with tens of thousands of dollars in debt when they could be starting families. Debts that will not be paid off until they start making real money in their thirties.
So we see two demographically destructive results of the demand for a college diploma for a good job.
1. Causing middle class families to limit the number of children they have to those they can afford to send to college.
2. Forcing young professionals to defer marriage and childrearing until they are out of debt and thus reducing their reproductive years to a brief 30-35 window.