The question has to do with whether we have an understanding of goodness that allows us to view suffering as a problem. Of course, infants are deemed innocent, then God doesn't have an moral motive to inflict suffering as well as a moral motive to alleviate suffering. However, if federal theory is accepted, along with a Reformed understanding of God's moral motivation to punish sin, then even an infant is under the federal headship of Adam and therefore God has a moral motivation to inflict suffering on the infant based on his justice, just as he has a moral motivation to alleviate suffering based on his mercy, and it's up for grabs whether he is just or merciful. Exactly the same situation obtains with infant reprobation as adult reprobation, and the only difference is that in the case of adult reproations, we have people who are visibly sinful, while in the case of infant reprobations, there is an appearance of the innocence of the victims. However, on the assumption that we can deserve punishment in virtue of our descent from Adam, this appearance is illusory, and there is no relevant difference between the two cases.
In ordinary human contexts, moral goodness/righteousness/holiness is centered around, among other things, the minimization of the suffering of others, or maximizing the benefit of others. This may not be all there is to morality (unless you're a utilitarian), but it's an important part of it at least. On the Reformed conception of goodness, this is a contingent fact about human beings in virtue of the kinds of social relations in which we find outselves. The Calvinistic response to the problem of evil maintains that this requirement for God to minimize suffering, either in the short run or in the long run, is a function of extrapolating the conception of human goodness, which is defined in terms of God's commands to us, to God himself, thus collapsing the creator-creature distinction and requiring God to minimize suffering.
That is why I have been arguing that the Calvinistic claim with respect to the problem of evil is to eliminate it, and this is done by rendering it a pseudoproblem. What the Calvinist claims is that God has a moral reason to inflict suffering as well as to alleviate it, therefore the amount and distribution of suffering does not count against theism one iota.
If this all seems counterintuitive to you, well, (and it seems counterintuitive on top of counterintuitive on top of counterintuitive to me) that's just a sign of how depraved you are, and how much you want to avoid God's justice, and how much you need to submit your intuitions to God's Holy Word. Or so I'm being told.