The biggest expense is college, at least in the U.S. If you retire before they apply, you may want to be conservative and set aside the cost of the most expensive college. MIT is $77k/year [1]. For two kids, that's $616k. And it increases faster than inflation, estimate 6%/yr. If you have enough money in the bank that you're financially independent, they won't qualify for need based aid, you'll have to pay the sticker price.

Columbia is more expensive.

And if one of them wants to become a doctor or lawyer, now you need to pay for grad school too.

[1] https://sfs.mit.edu/billing-repayment/undergraduate-tuition-...