A toddler dying of cancer falls under the category of "natural evil" (as opposed to moral evil, in which an intention or will is involved). You're correct to say that natural evil can sometimes be difficult to explain in a pedagogical sense. A storm taking one's home can be a way to teach us our dependence on God alone, but a child being taken by cancer does seem meaningless to us at the best of times, and simply cruel at the worst.

But the story of Job (in which his family, including his children, are taken away through natural evil) shows us two ways of engaging with this kind of evil: 1) Seeing the resulting struggle with God as its own form of growth, and 2) the virtue of humility.

In my opinion, there is no "one answer" to Theodicy - it requires all three and a healthy dose of Job's eventual humility. The first has been true in my life multiple times. The second is often helpful when engaging with evil on a cosmological level. The third is helpful when wrestling with the problem of moral evil. Taken together, I think the picture is reassuring, and certainly better than the alternative (a meaningless universe of hopeless suffering).

I recommend David Bentley Hart's "The Doors of the Sea" as a good short work on this subject!