The questions we must first ask in relation to our question:
Why is it worse for a child to die rather than a person of an older age?
Is anyone of any age owed even a day of life? Did we earn it by any merit?
Is it meaningless/cruel to give life to just take it away?
Compared to eternity, our life here is unfathomably short, whether it’s a day or 100 years, so we should not impose our idea of a long or short life on the situation, as a life in heaven is infinitely longer than any on earth.
It is widely acknowledged that God, in grace, applies the cross of Christ and its victory over sin to those who do not have the mental capacity to comprehend and respond to the gospel on their own. The Bible gives reasons for why this may occur, such as to save a person from greater evil (Isa. 57:1-2) or perhaps to bring about greater good in an unexpected way.
With our finite minds, we are unable to see any good coming from something as tragic as an infant death; however, from such a travesty, God can work to bring about a good outcome. We see this kind of "butterfly effect" in the Book of Genesis, where Joseph is beaten and sold into slavery, yet God works to make his hardships necessary to save thousands from famine and death.
Conclusion
The death of young lives is a stark reminder of our world's brokenness, and while we may not know for certain why they are taken, we can be certain that they will have an eternal life where such brokenness is non-existent and good reigns.