People with children in SF either have locked in cheap housing due to Prop 13, make top-2% salary, or move out. Like, seriously, SF proper has half the primary-school-aged children than it should. It's not just people not having kids - SF has approximately the "right" number of infants and preschoolers.
Under-appreciated fact about SF: your kid's schooling choices are either winning the school assignment lottery, having a 50 minute commute, homeschooling, or moving to a city that actually handles things sanely and has lower housing costs to boot.
Subsidized day care is $1900/mo for 2 kids; rent on a 3 bedroom house with a serious termite problem and an absentee landlord who doesn't care is $3300, babysitters are $20/hr if you want to go see a movie. Couples with 2 full time tech workers are fine (although they tend to not think so). I've got a spouse who works part time outside of tech and we make it work. We'll never buy a house here, and we had a landlord raise rent $1000 (33%) overnight once so you never really rest easy, but if you love your job it's a good place to be.
If you and your partner both work full time, factor 20k+/kid/year for child care. If you later send your children to private schools, factor roughly the same rate.
If instead you choose to send your kids to a public school that provides good educational opportunities, factor in either several thousand in additional local property taxes [1] or similar amounts in donations to the school PTA [2].
