Kids of (a conveniently 4 years apart) 10 and 14, and assuming we'll cashflow $2k a month to cover in-state tuition and living expenses. We won't have a mortgage by then and are saving a fair whack towards our FIRE anyway, so will just back off the savings for those years and let the pot build. We're planning to FIRE when the youngest finishes high school, so there might be some interesting 'oh look, we have no income!' aid calculations we can do then, but my plan assumes we'll be paying.
I have one small thing to consider though, and would appreciate input. My oldest is scary bright - tested recently in the top 2% on a national test, won a place at our local university's Honors college summer program for smart teens, etc. His current academic performance would win him an almost free ride scholarship at our largest in-state uni.
My youngest, on the other hand, so far appears more 'comfortably bright'; I'd estimate she's in the top third of her class, but not in the hugely stellar academic elite.
Would you spend the same on both? Oldest might only need living expenses, but youngest might need living expenses and full tuition; a difference, say, of $40k. At the moment, I'm thinking of equal amounts for both, regardless of need - if oldest doesn't need it for undergraduate, then he can have the money for graduate school or to take a year or two for improving travel, or to start a business, something like that (it wouldn't be available for cars and consumer tat, only life-altering experiences).