Originally Posted by xekomaya
This has been the teaching of the Catholic Church for over 700 years, and despite the milder language of the current catechism, it has never been refuted.
That's not quite true - the Church has taught that we cannot know the fate of the unbaptized for sure, children or adults.
As far as my views, the soul is present from conception - there is no such thing as matter without form, and the form of a living thing is a soul. A human soul is by nature an immortal, rational soul, even if much of it's abilities are in potentiality. So the fate of a miscarried baby has the same consideration as the fate of any other soul.
As for the question of whether the person you would meet in Heaven will be a "baby" still. Heaven is not the final destination of Christians. Those in Heaven are souls without bodies, enjoying the beatific vision. Without a body, I am not sure the designation baby makes much sense, though obviously the person will have had different experiences.
But the real end place is the New Earth, when all creation is remade and perfected, including our bodies. In this state, I think we will all be remade to fulfill all of our potentiality, so I think one could imagine that we will all be physically grown up and perfect in body and mind, according to our nature.
C.S. Lewis once observed (maybe in The Screwtape Letters?) that the vast majority of human life never lives a "full" human life. And that God must very much love and want all of those infants and small children with him for some reason. It seems likely that in the final equation, such people who never lived an adult life on Earth will make up the majority of the population in Heaven or on the New Earth.