Speaking as someone who has put one kid through college and still has another at home for 7 more years, here's some potential (ymmv) added cost:
- larger home/apartment
- health insurance increase
- out of pocket health care cost
- dental, best to budget for braces
- vision, best to budget for glasses
- clothes and more clothes. They are always outgrowing.
- larger grocery bill. If you eat out, larger bill at restaurants
- gifts, toys, electronics, etc.
- potentially larger vehicle
- perhaps more errands, so more gas money
- car once kid(s) turn driving age
- higher insurance if they are driving
- sports leagues, school field trips, camps
- bicycles that they'll outgrow every few years
- flying, trains, hotel rooms, beach rentals, etc. are all things that can add up quickly as you go from just you+spouse to a family
- college expenses/contribution (thankfully we will look somewhat poor to FAFSA in FIRE)
- wedding expenses/contribution
- entertainment budget - do you take them to movies, Disney World, gem mining (my kid loves this for some reason), etc.?
We are trying to FIRE in the next 6-12 months, so know what our current spending is, so in our case it is trying to decide how much of a bump do we foresee for things like a car/insurance and college, and then how much do we see our spending decrease after college. I'm not going to chose a number for the next 40 years that I'll only need for the next 11 or so.
FWIW, I'm using the added cost of one child as $7K/yr now and $15K/yr for the college years (state school plus some financial aid). That is with us already owning our home and vehicles.