My wife and I have lost a son, and I also spent some time addressing your question in my sermon on 2 Samuel chapter 12.
In my study for that sermon I found that Reformed and Calvinistic writers took the strongest position regarding the salvation of infants.
As I read your question, you are taking issue with the fact that infant salvation seems to be based upon their lack of opportunity to hear and respond to the gospel. It may be that the Q & A answer implies this. You rightly argue that if this is the crucial element, then many adults may not have heard the gospel, either. Why, then, would they not be saved on the same basis? Good observation.
In my mind, that isn’t the issue at all, however. There is a very great difference between an unbelieving infant and an unbelieving adult. Men and women are condemned to an eternity in hell because they have personally and willfully chosen to disregard and disobey God. When Paul condemns all mankind as sinners in Romans 1-3, justly condemned to an eternity apart from God, he does so on the basis of a conscious and deliberate decision. The heathen in Africa (for example), who have never heard the gospel, have seen the revelation of some of God’s attributes in nature, and they have chosen to worship the creation rather than the Creator. It is for this that they are condemned (Romans 1). The Jews, on the other hand, have an even greater guilt because they have had the revelation of the Law, and the full and final revelation of God in the person of Christ (Hebrews 1:1-4; 2:1-4; Romans 2). An infant does not have the capacity to grasp any revelation about God, and thus while born in sin, the infant has not yet committed sin by consciously disobeying or disregarding God (as adults have done). This seems to be the force of God’s argument when rebuking Jonah in Jonah chapter 4. Jonah wanted the entire city to be destroyed, children included, but God argued that they could not yet tell their right hand from the left. They had not yet reached the point of making a conscious choice to sin.
I understand that this is a very sensitive subject, but I think that the real issue is not the opportunity to hear the gospel, but the ability to choose to sin. While God has not given the revelation of the gospel to all mankind, He has given His natural revelation to all mankind, and unbelieving man rejects what He has revealed of Himself (Psalm 19; Romans 1). Because of this, God is justified in condemning all adults who have rejected Him to hell, whether they ever hear the gospel or not.
As I understand the Bible, men are involuntarily constituted sinners by their birth into Adam’s race. When Christ came as the last Adam, He overcame and outdid what Adam did (Romans 5:12 ff.). Thus, men (including babies) don’t go to hell for being the offspring of Adam, but for willfully, personally rejecting God and His revelation. Until a child reaches the point where he chooses to sin, the work of Christ delivers the child from the grasp of Adam’s sin.
This is a rather short and sloppy answer to your question, but a more thoughtful argument is to be found in my message on 2 Samuel 12.