I have long thought that speculation has a role in Christian theology in precisely those situations where someone says, "I can't imagine how that could be!" Sometimes those speculations end up being borderline heretical, or at least heretical if one assumes certain premises, and that has to be watched. At the same time, it may be better at least to have the speculations in one's back pocket for the time when one says to oneself, "How could that be?"
This sort of thing comes into play, for example, when talking about the Trinity. One will almost always say something heretical when one tries to get a clear concept of the Trinity, and I'm not going to brush off that problem. On the other hand, if someone says that the Trinity must be logically incoherent, a little speculation can at least be a way to argue that it does not have to be logically incoherent.
Most Christians of a philosophical turn of mind have given a lot of thought to the problem of evil, and I think it is highly biblical to use the concept of soul-making as one part of the answer to the problem of evil. In this thread I have been recently discussing that topic a bit with a blogger who lost his faith during a time of severe suffering.
To my mind, the harder cases are those where it is difficult to give (without speculation) a soul-making explanation of the suffering involved. These would be cases where the person suffering is an infant, young child, or mentally disabled person and is hence unable to process the suffering in such a way (it seems) as to be sanctified by it. It would also apply to cases where pain is so severe that it blots out thought. At least, these are problematic if we assume that soul-making is primarily a mental event--learning something, for example, or consciously clinging to God.
The trouble with saying that God uses these events as soul-making for other people is that that seems to mean that God isn't really seeking the best good of the suffering individual but rather is using him as a means to an end, which (in my opinion) is incompatible with the doctrine that God loves every person so much that he seeks that person's highest good. So, while it may well be true that God can use the suffering of an infant for good in the lives of the parents or doctors, that can't be the whole story. What about the baby? Those of us who are pro-life face related questions when we think about the babies who have died in abortions. What is God's plan for them?
Without in any way meaning to be flippant, I offer the following somewhat unusual speculations so that, at a minimum, we don't have to say, "I can't imagine what possible purpose God could use that suffering for when he allows it."
1) Mystical soul-making
What if soul-making isn't primarily a mental event or an event requiring conscious response, at least not for creatures who have souls and are intended (ultimately) to be rational creatures? This could mean, for example, that you could suffer while mentally deranged and somehow be purified by it, which would become evident when you were no longer deranged, even though you had no thoughts about it. And the same mutatis mutandis for infants, etc. I admit that this one is my least favorite of the speculations in this post, because it seems to me improbable that God deals with man in that way. The pattern that seems more biblical is of our response to suffering being the way in which God uses suffering in our lives, so that soul-making is not a process in which the soul is purely passive. However, I put it out there as a possibility, because that's the point of this post--exploring possibilities.
2) Levels of glory in heaven
Suppose we assume that all babies and those with childlike mental levels go to heaven. Still, it doesn't follow that everyone will have the same level of glory in heaven. The speculation here is that perhaps our sufferings here on earth are used, via our own response to them either here on earth or after death, to partly determine how glorious our individual heavenly state will be. This is a very Dantesque notion. The reader will recall how Dante has some in the sphere of the moon, still enjoying the presence of God, but in some sense lesser than those in the sphere of the sun. (It is from that portion of the Divine Comedy that the famous line comes, "In His will is our peace.")
3) Personal sanctification after death
Suppose that there are not different levels of glory in heaven (contra 1), that all babies go to heaven, but that each person has an individualized route to glorification. This seems pretty obviously true already, and Christians attest to their belief in this idea when they say that God has a plan for each of us, or God has a way in mind by which to sanctify each of us. Again, use the concept introduced in #2 that our own response to suffering after the fact, which might be after death, can be used in some way for us. In that case, the suffering experienced in this life by those who can't process or think about it in this life, for whatever reason, could still be used by God via our response to it after death to bring us to individual perfection.
I've saved the most heretical for last. Suppose that not all babies go to heaven and suppose that eternal salvation can be determined by what happens after death. Suppose that whether babies et. al. go to heaven depends on their own response to God after death when they are given the mental abilities of an older person. In that case, those individuals' response to knowledge of their own suffering here on earth, in conjunction with the knowledge of God vouchsafed to them at that time, could be part of what determines their eternal destiny.
I don't make any of these speculations lightly, and I don't know if any of them are true. I make them because the next time you hear someone say, "I can't possibly imagine how God could use this terrible illness this baby died of for the benefit of the baby," you should be able to respond, "That just shows that you need to expand your imagination."