Myself, I’m not very interested in purported naturalistic explanations for what Scripture so seems to proclaim as a divine intervention in history. They generally seem rather artificial. And anyway, the most profound aspect of early Judaism was the belief in a God who really does mighty deeds in real history, in real places, with real people, an aspect elevated further by the Christian idea of the incarnation.
Regarding the deaths of the firstborn, including children, we must remember that God is the master of life and gives a limited amount of it, in the biological sense, to any person in this age. We human beings do not have the right to take human life (ignoring complex situations like self-defense, war, and capital punishment), even our own life, because that life did not come from us and no one has authorized us to take it away. The hour of our death or of the death of another is the prerogative of God. For God though, He is under no obligation to extend our life forever, or even to the current biological limit of human life (about 120 years). Indeed, in his providential plan for history most people don’t live to anywhere near this limit; they are brought home, or to judgment, at some point earlier. This is what happened with the death of the firstborn. From the perspective of those left alive, it was a great plague. From the perspective of the Israelites whose firstborn continued to live, it was a great mercy from God and a sign of their covenant with Him (prefiguring, in an obscure way, salvation in the Christian sense). But from the perspective of those who died, including the children, it was simply their time, and none of them suffered unjustly because of it.
This is not to say human death of any kind isn’t in some sense an evil. It is. Death was not in the “original” plan of God for mankind. It’s only because of human sin that we are subject to this mortality. But God’s plan is over all, and no evil is permitted in it except so that a greater good may come from it. Inasmuch as the death of the firstborn was a physical evil, it was allowed to enter into the plan of God only so that something better may come from it, something that would not have been possible otherwise. And meanwhile those who died remained and remain beloved children of God. I imagine many of them are in heaven, and rejoice in how their short lives played some small part in the whole of salvation history, though not rejoicing in death itself of course.
