Posting mostly to follow replies.
To partially answer the OP:
we have 3 kids, We plan to move to a very HCOL: Tokyo (but we're flexible in how far from Tokyo we might want to live). Wife has been a Stay at Home mom for 5 years, and I plan to pull the plug in 2 years (or hopefully before that - technical my math says we're already FI but I'll need a job in Japan to secure a low-interest mortgage, before I can pull the plug).
Japan is not the US, but has similar constraints, in particular Tokyo which is roughly as expensive as HCOL areas in the US in my experience (education costs and housing* costs through the roof, health is expensive although controlled by compulsory national plans. I estimate we will have to pay 12k a year for national health + national pension).
I believe you have 2 choices: have a very crisp budget for the 20 years to come, or expect flexibility in life, which is how I approach things. I have not planned specifically for college costs for my 3 kids. However I have planned to spend a specific amount of money every year for them, which I know will fluctuate in practice. If things go my way, it will be cheaper in the early years so I can save a bit to pay more when they get to college. Worst case scenario, we can skip the international vacation once in a while to get back on track financially if needed, or cut on other expenses, move to some European country where education is virtually free, or, god forbids, go back to paid work at some point if my calculations turn out to be off by 5 digits.
I think flexibility is the key as you add members to your household: each new member adds lots of variables to your life, so your plans basically need to adapt to that. With more kids, you statistically increase the risk that one of them will have expensive medical costs. Incidentally, you also increase the chance that one of them decides to leave home and start a career without going to expensive college.
You've mentioned you're committed to the Bay Area. Do you have details? Not that I pretend I could change your mind, but I'd love to try (from my perspective the only reason to stay in a given location would be family. Anything else is negotiable)
* our 700sq feet condo cost us $500'000 in Japan, when we lived there before. And that was in the suburbs of Tokyo, not Tokyo itself. I don't know of many families of 4 in the US who would find it ok to live in 700sq feet. Again, flexibility.