Among our species, the optimal child bearing years are about 30 years before the peak earning years. If we waited until we could afford families, we would become extinct (as, by the way, appears to be happening in many secular cultures).
Having no money is a nuisance, but one can manage. Medicaid covers most medical bills. Assuming you don't plan on home schooling, you can usually find some school if one is still poor when the kids reach school age. The more frum the school, the easier to work a deal if you are poor. It's been a VERY long time since the typical poor person in our community had a one room room apartment (back then, the parent's bed typically had curtains - and no one else had a bed). Especially in "blue" states, there are plenty of welfare programs in addition to programs such as "Food stamps" and "WIC."
Remember that this is the first time in history where a major public health issue among the poor is obesity. Poverty today more often means having fewer luxuries than the neighbors. Note how people lived 100 years ago (tiny housing, no private cars, no air conditioning, often no central heating, no antibiotics, severe discrimination against orthodox Jews, and if you lived in Europe, frequent pogroms with the worst about to happen), and don't whine.
It should also be noted that the marginal cost of each children is quite low. Schools (both ours, as well as universities) pay attention to family size, Adding one more place at the table costs little. And by the time kids reach middle school, they probably are doing a fair amount of work (especially in poor families).