Why do you believe that infants who die go to heaven?

I believe it not because of a sentimental notion that babies aren't participants in the Fall. They are. Babies are participants in original sin.

The question is whether God has a way to cover their sin even before they have a chance to believe. Babies are not mentally able to put faith in Jesus yet, at least not in any terms that we ordinarily understand. And so I think that God provides another way to cover their sin.

I base my belief that God does not condemn babies who die on Romans 1:19-20:

For what can be known about God is plain to them [that is, to mankind] because God has shown it to them. For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made. Therefore, they are without excuse.

The "therefore" at the end says that mankind would seem to have an excuse if they had not seen clearly in nature what God is like. And so, because I don't think little babies can process nature and make conclusions about God's grace, glory or justice, it seems they would fall into the category of still having an excuse.

The way I see it is that God ordains, for his own wise purposes, that at the judgment day all the children who died in infancy will be covered by the blood of Jesus. And they will come to faith, either in heaven immediately or later in the resurrection. And God will not condemn them because he wants to manifest openly and publicly that he does not condemn those who did not have the mental capacities to put their faith in him.

I don't think imbeciles have that capacity, and I don't think babies do. Therefore I hold out hope to parents that the loss of an infant is not their eternal loss.