it probably much more than $15k given it was just a starting salary and probably other perks too. Also a lot of financial aid and other govt. programs for parents.
In my own case, having a single toddler is about a 20% increase in expenses, and that's not even accounting for saving for college, or how much it would cost if we wanted more space. It's easy to imagine being relatively comfortable in the Bay Area but not feeling financially comfortable about having kids.