Answer: if they were baptized.
One of the main beliefs of Christianity is that humans are born sinful, as they have inherited the guilt of the original sin of Eve. When children are born, they need to undergo baptism in order to signal their allegiance to the Church. People who are baptized are accepting the existence of God. When a child dies without being baptized, he cannot join God in heaven. He also will not resurrect in the day of the Final Judgement. This is part of the reason why medieval theologians were so concerned with the baptism of children.
