The Church does not teach that the souls of the unbaptized go to Heaven, only that we hope they might
On the one hand, the Church teaches that God is just; on the other, that we need to be baptized to get rid of the original sin on our souls. Thus, what God in His justice might do with any uunbaptized soul is not revealed.
The theory of Limbo was taught for a long time, and now it is taught more clearly that what happens to unbaptized souls after death has not been revealed. It is for this reason that we can have hope.
We mourn for the victims of abortion for Tha same reason we mourn anyone who is murdered, because they did not have the chance to attain the level of sanctity God had planned for them.