We're not quite at the RE part (though well past FI). The thing about kid expenses is that they can last as long as you want them to -- or stop whenever you want them to. So the first question is what do you and your spouse want?
College: what will your EFC be, based on your current income and the number of kids who will be in school at the same time? This is key if you intend to FIRE before they graduate, but even if you plan to wait until the kids are out of school, your EFC will likely affect your savings/savings rate over that 8 years. Figure out what your EFC is -- then figure out if you want to be able to contribute more if it allows your child to go to some "dream" school.
And don't forget about the other expenses: you'll likely continue to cover their phones/medical/car insurance until they graduate, so account for that. What about if they want to join a fraternity/sorority? Will you cover that? What about things like the cost of a new laptop, travel to/from school, clothes, spring break, etc.? Do you plan for them to cover their own incidental expenses/nights out from a part-time job, or do you plan to give them an allowance through college? How about living expenses if they get a summer internship where they need to rent an apartment (my nephew spent last summer in San Francisco, and this summer in Manhattan!)? What about grad school? There is no right or wrong answer. The only thing that matters is that you and your spouse agree, and that you account for those agreed expenses.
Also note that those expenses can extend well past college, if you'd like them to. Some of my DD's friends couldn't get jobs in their fields after graduation and so moved home to work at 7-11 for a year until they found a job. Most kids also don't have the cash to cover moving expenses and apartment security deposits and such. If they move away, do you want to pay for plane tickets for them to come home for the holidays, or join you on vacation? Again: no right or wrong answer. Just talk through all of this stuff with your SO, repeatedly, until you're on the same page.
And then be prepared to change your mind once you're actually in that situation. ;-) AMHIK.
I will say that for us, I have very much enjoyed having enough extra money to be able to fly my DD to join us on vacation without batting an eye. The thing that I realized when she moved halfway across the country for her job was that I now need to bribe her to spend time with us instead of with her friends and activities -- and I'm realizing why, when our kids were younger, every set of grandparents did periodic big family vacations where they either rented some giant house or planned a trip in some incredibly tempting location. ;-) I am very much looking forward to being able to do the same, and so it is really a privilege and a pleasure to be in a situation to do so.
Oh, and that totally-independent DD has still managed to delay delay delay getting off our car insurance and cellphone plan. ;-) We could push harder if we wanted or needed to, but again, it's nice not having to worry about stuff like that.
