i'm not sure the divide by members of household makes sense past a couple. kids dont add to living expenses to the point that its as much as the couple costs. kids in general should be a very minor blip on your expenses when you're FIREd and dont need daycare etc. food / clothes / some activities, am i missing something.
medical insurance and care. Dental too. Maybe a car or larger car. College or school expenses for future. Clothing. Hobbies. Higher utilities due to use (or would that not count as its a "housing" expense like a bigger house because of needing more bedrooms than a couple wouldn't count for this poll).
Here's some data from the USDA:
"Based on the most recent data from the Consumer Expenditures Survey, in 2015, a family will spend approximately $12,980 annually per child in a middle-income ($59,200-$107,400), two-child, married-couple family."*
This statement implies that average spending varies from $16,620 per adult, and $12,980 (78%) per child to $40,720 per adult, and $12,980 per child (32%). However, they also note the following:
"Not surprising, the higher a family’s income the more was spent on a child, particularly for child care/education and miscellaneous expenses."*
I expect the $12,980 per child more closely coincides with the average middle income family, so $28,670 spending per adult and $12,980 spending per child (45%).
*https://www.usda.gov/media/blog/2017/01/13/cost-raising-child