Childhood morality is a difficult issue, both in theodicy and pastoral ministry. Why doesn't God stop it? A few quick observations:
i) Sometimes kids turn out badly. Their life ends in disaster. Suppose a child goes to haven when he dies. Then he doesn't go offtrack.
ii) Some kids turn out well, but their life may adversely impact other lives through no fault of their own. Let's say Stalin's mother was a wonderful person. But indirectly, her life caused untold suffering for tens of millions of Russians under Stalinism.
iii) It might seem like a child who dies in infancy (or by miscarriage) is a wasted life. There is, however, an all-important difference between existence and nonexistence. Once conceived, the person will live forever. In relation to this life, they were snuffed out tragically early, but in relation to eternity, that's infinitesimal. What ruler are we using: this life or the afterlife?
iv) Child mortality makes room for other children. Additional lives. Although that may sound like a cruel way to put it, they aren't simply squeezed out to free up a slot. For death is not the end, but a new beginning.