This is one of the weirdest questions I have ever seen anyone ask. Besides the obvious time issue...uh...why would someone earning less than the poverty line with no guarantee for a job in 6/7 years have a kid?
Many graduate students have employed spouses (as I did) and reasonably competitive fellowship packages (as I did). Many of us took time off between college and graduate school, worked decently-paying jobs, and managed to put some money away (as I did). Many of us, seeing the struggles of TT junior faculty with young children, felt that it was far saner to have children when our schedules were far more flexible and there wasn't a ticking tenure clock. Many women--and presumably men, too, although with less biological urgency--simply don't want to wait until their late thirties or early forties to have children, for a variety of reasons.