Yeah, historically people had lots of kids not least because lots of them would not survive...
As for today, child mortality in developed countries is well less than 1%. And most of that is infant mortality (death before age 1). For example, http://apps.who.int/gho/data/view.main.CM1320R (sorry, somewhat slow) shows that in the US in 2015 infant mortality was 5.6 per 1000, while child mortality was 6.5 per 1000. Neonatal mortality (death within 4 weeks of birth) was 3.6 per 1000.
(Insert here rant about how different countries report some of these numbers differently, with some counting a 21-week birth followed by death a day later as a miscarriage, some counting it as a stillbirth, and some counting it as a live baby that died. As the mortality numbers drop these edge cases matter more and more. Obviously back when child mortality was in the mid-double-digit percentages these edge cases were not really important.)
What was your naive assumption about child mortality rates, if I might ask?
I don't remember having a concrete number in my mind. I guess I'd assume .1% for today. Had a picture of it as "something that's very tragic and basically doesn't happen anymore". While .5% puts it into the "not likely but might happen to you" range.
And as for a century and two ago I'd be waaaaaaay off base. Probably 10% or even 5%.
It's hard to say in the hindsight. Now it seems so obvious and makes you feel stupid.
