Marcel Nguyen Tan Van, the Church and the Salvation of Unbaptized Children

You refer to the reported private revelations of Servant of God Marcel Nguyen Tan Van, in which Jesus reportedly told him that all unbaptized children are saved.

In short, the Church does not rely on reported revelation for her official teaching. Indeed, even approved private revelation cannot add to the Church’s deposit of faith, which is contained in public revelation, “the Revelation of which Christ is the fulfillment” (CCC 67; see 66-67; 84).

As provided in her official teaching, based on the sacred word Jesus communicated to his apostles, “The Church does not know of any means other than Baptism that assures entry into eternal beatitude” (CCC 1257). And pertaining to your specific question, the CCC provides:

As regards children who have died without Baptism, the Church can only entrust them to the mercy of God, as she does in her funeral rites for them. Indeed, the great mercy of God who desires that all men should be saved, and Jesus’ tenderness toward children which caused him to say: “Let the children come to me, do not hinder them,” allow us to hope that there is a way of salvation for children who have died without Baptism. All the more urgent is the Church’s call not to prevent little children coming to Christ through the gift of holy Baptism (CCC 1261, emphasis original, footnote omitted).