Many years ago, my sister mis-carried in her 7th or 8th month, as soon as that baby's skin started to show, before it (he) was even fully out of the womb, the nurse, anxiously waiting, instantly baptised that infant because ya just never know. Well, perhaps these days they *would* know if and for how long the baby had been dead - but we never knew for sure if that baby made it to heaven or not - we never believed for sure that he made it but under the circumstances, to this day we hope God saw fit to allow him to be baptized before taking his soul.
If God saw fit to wait for that baby to be baptised before taking him, that baby is without one doubt, happy for all eternity in heaven, hopefully with my sister - if God however did not wait but instead took him hours or days? prior to the actual baptism, then we never believed that the baby went to heaven because no one makes it there with original sin - thats why baptism is so important. Its black or white, I fail to see that there can be another answer. We are all conceived with sin, no way around that, if God chooses to grant those infants with original sin into heaven, he never made mention of it and the Church has been cautiously quiet about it for a long time.
We simply cannot know why God would take a helpless baby out of this world at all, with or (God forbid) without Baptism. He has His own reasons - and we have hope, beyond that, we can only surmize our thoughts that perhaps in His mercy, He knew it was better to take that child out of this life than to allow him to be baptised then sin his whole life and end up suffering in Hell. That too would be mercifull. We will never know that till the next world.