John MacArthur famously used 2 Samuel 12:23 where King David said of his dead infant "I will go to him but he will not return to me" to justify his belief that the unborns and newborns will automatically be saved. Some even willing to use Jesus' remarks of little children as another proof of such, which MacArthur rejected.
I think most reformers do not jump to such conclusions. Because not only it does not affirm biblical inerrancy, it allows emotion to cloud judgment and discernment.
For example, those who are caught in or confused of such emotional belief often respond to critics with the statement "God is always just", "God will not do wrong", etc. This is off topic. And whenever someone reached the limit of their logic without humility, they will steer off topic. Worse, should they not admit better discernment from others nor their jealousy, they would have to shout to others: "Don't be TOO PROUD!" Not surprising, such is the case with what their "Fathers" did to our Lord when he was on Earth: Blasphemy! You are NOT GOD! Don't be too Proud!" But actually, they meant "We are God! So how can you be God?!"
Anyway, the strongest point for instant paedo-salvation is 2 Samuel 12:23. So I will deal with that verse, while I agree with MacArthur, that verses on how Jesus love little children are irrelevant to this issue.
First, David had been wrong before (1 Samuel 25:22) even when he invoked God's name. So, biblical inerrancy does not mean we read without discernment.
Secondly, "I will go to him..." does not mean go to heaven nor share the saving grace. The likes of John Calvin are more careful in dealing with this meaning.
Thirdly, "but he will not return to me" surely David is wrong yet again. What of resurrection of the dead? And I'm not even talking about the granting of eternal life. If David was aware of God's power, it's more likely that he was referring to the experience of death and not heaven and this world.
So therefore, this is not as clear cut as they would like. However, unlike some, I would hold that it is crucial that the only way to eternal life with God, is through the cross of our Lord. It is so for Enoch, Elijah, and every infant. Those who even dare to consider this unnecessary and say "God is God, God is never wrong, God is just" is betraying their own logic. What then, is there one man so fashioned by God that he needs not the cleansing spirit and blood of Christ upon his soul? Is there such soul that is not sinful by default? even before birth? Or is the redemption of Christ so weak that it is constricted by time and space? The only way to walk this path of belief is to follow strange ways. The sinful nature of such ideology is interesting to investigate but this entry does not provide enough room for such treatise. Another time.