Housing: children used to be expected to share rooms. Now it's not the norm -- but there's no reason it can't become the norm again. When you talk to your parents or grandparents, they often had large families living in tiny houses.
Daycare: children used to be tended to by family members or religious communities. Now we isolate ourselves from family by moving far away, don't join communities (religious or otherwise), and for whatever reason don't ask mom and dad for help with the kids. Also, the age at which we trust children to stay at home without supervision has risen over time. These are all choices we're making that make children much more expensive.
Education: k-12 is free in the US, but lots of folks want private education for their kids. That's fine, I do too, but I realize it's not a required cost. University is definitely expensive, but loans are widely available. And, of course, not every child needs to go to an elite university.
These are the choices making it difficult to afford children, but they are mostly choices we don't have to make.