@carlsen – I think you need to decide in advance that you are willing to spend x on college, regardless of the college choice, and you need to use the same framework for your younger children. You should bump the dollar figure up for the younger ones to reflect the increase in college costs over time.
You will start down a slippery slope if you announce that you will pay for MIT or CMU but not for Cornell or Tufts (or whatever schools you want to sub in).
Suppose your next children do not have the academic qualifications for admission to MIT or CMU. Do you strike a different arrangement for them?