The global mortality rate of infants and youth up to the age of 15, based on an average of many studies, was almost one-half (46.2%) for the two millennia up to about 1900. By 1950, it was 27%. By 2017, it had fallen to 4.6%,.
The global infant mortality rate for children under the age of 1, again based on the average of many studies, was more than one-quarter (26.9%) for the two millennia up to about 1900. By 1950, it was 16%. By 2017, it had fallen to 2.9%
Here's a figure showing the patterns from Max Roser at the "Our World in Data" website (June 11, 2019). Of course, you will need to expand the version here, or go to the other website, to see the details.
I'll put off all the arguments over reasons why this happened and what it means for public policy for another day. It's Father's Day today, I just want to take a few minutes and marvel at this fundamental change in what it means to be a parent in the 21st century, especially in a high-income country. My children were much less likely to die.