Explain it as if they were children. Here's what I have told my young children and they can understand easily.
Little babies who die before baptism go to a place called Limbo. It is a special place of only happiness. But it is also not possible to ever be with God because only those who are baptized can be with God.
This is what my 3 y/o and 5 y/o understand.
They will probably still adamantly disagree with you, but this is as much information as they can grasp. Because of their emotional and spiritual immaturity, I think it would be charitable to explain it to them as you would a young child.
I agree 100%. I saw someone get thrown out of Freerepublic for teaching this dogma of limbo of infants. The Catholics at FR wouldn't hear anything but that unbaptized babies to go to heaven.
They posted the CCC 1261:
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.
The thrown out poster correctly pointed that CCC 1261 does not say that the unbaptized infant is saved, it just says that "can only entrust them to the mercy of God", which says nothing one way or another. He pointed out how God's mercy is the natural paradise of limbo of the infants, but no beatific vision. He was booted out. They don't want to hear anything "negative". Never mind that it is the teaching from the beginning, and the Vatican II church has never taught that unbaptized infants are saved.