Like political theories (libertarianism comes to mind), many theories of the Atonement founder on the rock that is the existence of children.
This is especially true of some of the more “reasonable” theories of the Atonement, like the exemplar theory or like mine own Meaningful Choice theory of the Atonement. None of them really explain how it is that Christ needed to atone for children before they were of an age to repent. I suppose you could fit them in with enough verbal gyrations, but they don’t really work.
One lesson here might be to not take any one theory of the Atonement too seriously. The Atonement is the thing, not the explanations of it. If a theory illuminates some aspect of the Atonement, that’s enough. Comprehensive theories must await the exaltation of our minds that would enable us to comprehend them.
Another might be that Mormons should not get too huffy about the advanced reasonableness of their belief over that of traditional Christianity. Mosiah 3‘s explanation for the Atonement of children, and, indeed, its explanation of the Atonement in general, is more or less traditional Christian mysticism about the Incarnation.