Well, people in the US made way more children when they made very few money, and lived 10 together in tiny houses and apartments.
And they still do in the developing world.
So it can't be just costs.
Our expected spending on children has also grown dramatically. For example back then there was an expectation that someone would always stay home an look after the kids, now we expect them to go to day care. Back then 4 kids sharing a room their entire childhood was normal, now each kid is expected to get their own room. Back then kids where expected to entertain themselves, now we expect them to attend all sorts of after school activities. Back then they where expected to either go straight to work after high school or pay their own way through college either by working or scholarships, now the parents are expected to help pay for college, and so on.
Basically it's not that having kids per se is more expensive, it's that providing kids with the quality of life we've come to be expected to provide them with is expensive.
In the past, kids could work on the farm or in a factory and generate money, now you have to send them to daycare and buy them toys and they cost money.
Previously, more people would jump straight out of high school into blue collar jobs and raising families. Now, men and women are delaying kids to focus on college and building a career.
And mothers would more often stay home to raise kids. Now both parents are working more, and there's less time to spend raising children.
Boarding houses have become illegal, and their modern equivalent of micro apartments are in danger of becoming illegal [0].
To add a personal story, when I was college we had about 20-30 people living in a medium sized single family home. We were mostly students, but there were a couple of kids, and a grandmother too. Our neighbors hated us, and constantly tried to report us for overcrowding. However in our locality there was no cap on the number of residents, so long as conditions are sanitary and safe. But we'd have the zoning person come through frequently to ensure we didn't have things like too many ovens or broken bathrooms.
0: http://www.sightline.org/2016/09/06/how-seattle-killed-micro...
How many bed rooms and how did the actual sleeping work?
A "standard" 3-bedroom house would have 7-8 people in it per room (assuming you use the family room as a bedroom too) which means either bunk beds or wall to wall sleeping bags? Also 30 people sharing two bathrooms doesn't sound fun.
First world problems...
Most people don't live with grandma anymore, and the government will happily take your children away if you don't keep every second of their lives supervised by an adult, prior to the age of 13.
Today's mom, on the other hand, moved to San Francisco to get her data scientist/devops/whatever job (there were not any in her area of northern Minnesota and she kind of liked CA after going to Stanford) and Grandma doesn't want to move to CA because it's going to fall into the ocean, you know.
Back then: Have kid when 18-24, Grandma is 36-42.
Now: Have kid when 35, Grandma is 70.