I don't know when I'll have the time to give you a researched answer, but put yourself in the shoes of a gen x and think about those 2 items you mentioned - higher ed and childcare. Birth rates had been in decline while the higher ed markets (and housing) heated up, but there's a very clear reason the demo cliff is hitting higher ed next yr- the great recession. Think about how the timing of these events impacted those in each generation.
Gen x navigated inflated higher ed then just when they got the house- Charlie Brown and the football. Your housing price just tanked and who knows about your job, whether you still have it or the promotion you'd been working for disappeared. You don't have a nest egg to fall back on since you spent a disproportionate amount of dollars on education / down payment and now you have those bills ahead? Looks like we're not having kids, or at least less of them.
We claim to covet an educated populace, but these barriers and their timing ran counter to those goals then we didn't do anything about it.
Oh that's easy, because I am in those shoes! I was born right in the middle of the '65-'80 Gen X range. But I would argue that the skyrocketing higher ed costs came after our generation was through and has hit Millennials and Gen Z right in the face - I think I paid $3,000 a year for school in the early '90s, equivalent to about $6,200 today. But I paid my way through the last two years of school so did graduate with some student loan and credit card debt, which kept me from saving/investing in my 20s but I got by. But I wouldn't call it crippling, despite all the $29 late fees I paid on that credit card debt.
At 31 I had a kid, and at 32 I bought a condo. But there was no down payment needed - it was the heyday of 5/1 interest-only ARM loans that took 15 minutes to sign up for. Of course 5 years and 1 day later my condo was worth $250K less than the loan we had on it and I couldn't afford the adjusted payment. And oh yeah, on top of losing the condo I did lose a job during the GFC.
I did pay exorbitant daycare costs from 10 months old until she was 5 and went to kindergarten.
So yeah, the start of this century was pretty financially rough for a lot of our generation, and it sure was for me. But all that said, our decision to not have a second kid wasn't financial at all.