These two posts are not incompatible. That 13.5% of kids needs to live somewhere. I'd bet that a heatmap would show "kid locuses" in some neighborhoods and "kid voids" in others.
They are not incompatible only if by "tons" you mean "half as many". I would expect some areas of San Francisco to have more kids than others but overall the number is very low.
SF is a small region in a larger urban area. My guess is that you could easily single out dense, urban sub-regions in LA or Chicago that are as childless as SF. SF used to have more kids, but I'd guess that this used to be true of the dense, urban sections of other cities. The difference is, a kid who moves from urban chicago to suburban chicago is still counted in Chicago's stats.
Meanwhile, some districts in SF might have quite a high percentage of children compared to the average for areas of equal density. Not sure of this, but it would be interesting to see.