In regard to the age of reason, it is the goal of the CHurch, as I understand it, to discenr the zaverage age at which children become truly aware and and grown up enough (though obviously they aren’t yet adults) to take responsibility for their actions.
I don’t remember any such personal epiphanies, myself. But I know others that do. Some of us come to this recognition in stages, others through a particular event. The actual age for each person may vary quite a bit. The Church’s understanding is that before this point a child’s life, we are incapable of giving full consent of will to commit sin. This means that, though venial sin may occur, mortal sin is impossible. So, we believe that baptized children that die before the age of reason are saved by virtue of their baptism alone. Nothing else is needed.
In the case of unbaptized children, the Church reminds us that though God has bound our salvation to the sacraments, He Himself is not bound by those sacraments, and we entrust them to the mercy of God. Certainly we do not expect that a God of such boundless love and mercy would forget these little ones, especially those who die unbaptized because of miscarriage or stillbirth, or because their parents simply did not understand the Church teachings.