What does modern agriculture have to do with infant mortality? It makes sense to exclude infants, because the reduction of those deaths are likely due to improvements in modern medicine and personal hygiene.
The thread above was not about modern agriculture; it was about the pre-historic time when agriculture was adopted and the availability of more food enabled more children to survive.
Infants eat breast milk, and if the mother does not have enough to eat, there is not enough milk. Sufficient and stable nutrition for mother is important to avoid child mortality.
In context of ancient world, nutrition of the family also had other mechanisms that impact the survival of children (strength to build housing, or strength to defend against hostile people, etc).
I still did not write about modern medicine; I don't know why you bring that up. This was about the transition from hunter-gatherers to farming, first in Mesopotamia and a few other places, about 10 000 years ago or so.
That was the "advent of agriculture", also known as "Neolithic revolution".
However, overall I'd say that starvation vs. having sufficient food impacts mortality rates more than modern medicine. You can't eat the best of hygiene and medication doesn't nourish you.
