What point is there in baptizing children before the age of reason then?
Original sin still exists on those childrens' souls. No, they did not themselves merit damnation, yet they are not entitled to Heaven.
Very correct, and in the case of those who are not competent to declare intention of receiving the Sacrament of Baptism, the intention of another is the proxy.
However, in situations such as a miscarriage, there is potentially no opportunity to confer the Sacrament though fully intended. This leaves us with a conundrum, as the Sacrament's conferring is impossible. As the one who is Justified, the parent, intends to confer the Sacrament, it is potentially true that their intention, though the ability is lacking, suffices in sort of a Baptism of Desire by proxy, just as actual Baptism is done by intentional proxy.
While this is not defined by the Church, it is a possibility.
To take your argument to its extreme, we end up with the Protestant conclusion of excluding infant Baptism. To take the other argument to the extreme we have Universal Salvation. The key is a middle ground and gray area wherein we act in Faith and hope to obtain the Mercy of God for another.
"For God does not command impossibilities, but by commanding admonishes thee to do what thou canst and to pray for what thou canst not, and aids thee that thou mayest be able.[58]" - Trent, Session 6, Chapter XI
http://www.ewtn.com/library/councils/trent6.htmThe safest thing we can do is pray and hope, holding to the doctrine on Original Sin, not declaring for certain all the little babies are in Heaven, and neither can we say for sure they are all lost in light of the Mercy of God and the reality of a valid intention minus the Form and Matter.
It's admittedly not an opinion which is rigorous in its application, but it is a pastorally sound method which does not do injury to doctrine as it leaves it up to God.