I admittedly have never lived in SF with kids, but this seems like a series of choices and likely some exaggerations. If one of you is making >$500k and both of you chose to work, have kids, and you don't have the option to shift morning/evening so nanny/daycare is only 40 hours these are all choices.
Even at the rate you quoted it doesn't seem unreasonable to be able to stash away significant amounts of money. The problems are very similar, but the solutions are a lot more enviable than most of the US.
4k a month is probably average for an over the table nanny at 45 hours a week with 1 kid.
Au pair is an inexperienced person which is fine, but not for the type of nanny work someone may need if they are working a lot of hours.
I believe you should be able to save serious money at 500k if you dont over extend yourself on a large apartment, but things add up very quickly so I can see how it might not be much left over with 2 kids.