There are several trusted sources on the Doctrine of Infant Salvation.
This link is very good on the subject. Spurgeon is almost always gold in everything he writes or says.
This link is excellent on the Doctrine of Infant Salvation and is probably the largest study on the subject that I know of. The author B.B. Warfield was one of the early Princeton theologians.
And there is "The Reformed Doctrine of Predestination" by Boettner. This, I think, is the standard work on Predestination and he treats of the Doctrine of Infant Salvation in that book. The following link should take you to that part.
As to my own understanding that infants dying in infancy are saved, it is based upon the above and certain Scriptures.
"[David] answered, "While the child was still alive, I fasted and wept. I thought, 'Who knows? The LORD may be gracious to me and let the child live.' 23 But now that he is dead, why should I fast? Can I bring him back again? I will go to him, but he will not return to me." (2 Sam. 12:22-23, NIV). Thus if we assume David to be saved, then he will meet with the infant in Heaven.
"Jesus said, "Let the little children come to me, and do not hinder them, for the kingdom of heaven belongs to such as these."" (Matt. 19:14, NIV). See also Mark 10:14, Luke 18:16. Here we see the compassion of the Lord Jesus upon little children.
Revelation 7:9 also supports this view, "How can there be in heaven a countless number of people from every nation, tribe, people and language (Rev. 7:9)? Surely not every tribe of people around the world has adult believers. Is it not possible, therefore, that a number of tribes will be represented by children who die in infancy?"
There being a "multitude that no man can number", seems to be a stretch if we consider the number of adults who profess Christ in any generation. But those dying in infancy since Creation would indeed be a vast multitude.
Jonah 4:10-11 "But the Lord said, “You have had pity on the plant for which you have not labored, nor made it grow, which came up in a night and perished in a night. 11 And should I not pity Nineveh, that great city, in which are more than one hundred and twenty thousand persons who cannot discern between their right hand and their left—and much livestock?” Now it would seems highly improbable that Nineveh had 120,000 mentally handicapped people running about that could not tell their right hand from their left. Rather it seems more likely that there were 120,000 infants upon whom the Lord would shower His mercy, for infants cannot tell their right hand from their left.
It would seem that if there are more people in Hell than Heaven, then the Devil has won the contest. I do not think he will be able to gloat on the Last Day for a milli-second when the full number of the elect are gathered around the Throne of the Lamb. God will get the glory in the numbers He has saved.
It gives great comfort to parents whose little ones have died.
And should indeed be a great spur to those same parents to believe in Christ, so they will see those babes again and not stand condemned by them!
The doctrine has implications on some other practical issues like contraception, covenants etc, but that might be a discussion for another time.
Anyway these are just a few thoughts on the topic.