Where do the children who die without being baptized go?
Catholic doctrine teaches two things about it:
1) That children who die without baptism cannot go to hell because they have no personal sins.
2) That the only means the Church has to forgive original sin is baptism.
Harmonizing these two truths, traditionally many theologians used to solve the question by saying that children who died without baptism did not go to heaven or hell, but to a place of natural happiness, but without vision of God, called limbo. There was nothing in the revelation to support this view; and it was never a dogma of faith, but a common explanation in many theologians.
The International Theological Commission studied the subject. The conclusions are that there is nothing in the revelation to sustain the existence of limbo. And that while the only means the Church knows for the forgiveness of original sin is baptism, that doesn't mean God can't forgive it. So, trusting in God's mercy, the Church can expect children who die without baptism to go to heaven.
While we have no data revealed to confirm this teaching, there is also no data revealed to reject it. And it is the one that seems most consistent with God's mercy. So, let us be comforted by knowing that these children are in heaven.