The whole thrust of the Genesis story is that Adam is created as the head of the human race and the lord of all material creatures, and Eve is created as the mother of all living things. (At least on Earth. Maybe in the whole universe.) They had the spiritual power to name and tame, and thus to affect how every creature grew and developed.
Because they were given so much power by God, their shared sin affected all living things (at least on Earth).
This is not a matter of God changing things; rather, this is God allowing Adam and Eve to suffer the consequences of their own acts. But it is also a matter of everything else in the universe, and every later human, suffering the consequences of Adam and Eve’s acts. There is something in us that protests against death, even the deaths of animals, because we feel that death is somehow unnatural. Genesis tells us that death really is a bug, not a feature.
(Obviously in the universe we’ve got, death has been made somewhat useful to the natural world. But most creatures seem to have senescence and death features that turn on, which implies that they could turn off. And there are other weird things like that. Cue Twilight Zone music.)
Obviously everything dying, because two humans wanted to be like God and were willing to steal for it, is not “fair” in a one-to-one individual consequences of sin way. But it is exactly the way temporal consequences play out, and particularly how historical acts of leaders play out.
It’s not my fault that sailors killed all the dodo birds, and it happened long before I was around. But it still affects me. I’m never going to see a living dodo bird, absent some DNA finagling, and I can’t make the slaughter never have happened. I don’t get a vote on history that’s already happened.
So of course babies have not sinned. But they live in a world which Adam and Eve refused to rule justly, and therefore there are things like disease and weakness of body and birth defects. Baby death is not a matter of fallen human nature; it’s a matter of all Nature having been trampled and broken by Adam and Eve’s sinning feet.
And that’s why we have this in Romans 8:19-23:
“For Creation is awaiting the revelation of the sons of God; for all Creation was unwillingly subjected to empty futility. But having been subjected, she is in hope through Him, because Creation herself shall also be delivered from the servitude of corruption, into the liberty of the glory of the sons of God. For we know that all Creation groans and is in labor until now, and not only she, but ourselves also…”
- ktisis, Creation, is a feminine word in Greek. That’s just grammatical gender; but it makes more sense to say “she” if she’s metaphorically pregnant and having birthing pains.