1-Are you calvinist or armenian (I lean toward this view) or something in between, cause knowing this might help a lil.
2-Do you think God doesnt know the heart and whether or not a child will accept Christ in its heart (or be predestined), when God knows all possible futures? How can someone's name be written in the book of life BEFORE they were made? It is because he KNOWS what the person will choose before they do, even if they dont get a chance to choose him in this life, he still knows their hearts and whether or not they would choose him, if given the chance. If God does save all children who die before their age of accountability that would mean that God KNEW they would choose him and so he lets them in heaven. God does in fact choose where people are born and he knows who will never get a chance to live, so he puts into the body of a child that dies, a soul that would choose Jesus and wala, they = saved.
3-Will God let unrepentant sinners into heaven? I dont care if someone is never born or what not they are still sinners and not holy. We have all sinned through Adam, so we all choose his road willfully. If this is so, then God cant just allow a sinner in heaven. Him doing so is not grace or anything that is Godly. He made the rules and eveyone plays by them, even himself. We just might not see everything God does, because we have mental problems like: What happens to children that never get to live long? This is a common question skeptics ask Christians, to stump them. Well, they either go to hell or heaven depending on what they have eternally choosen: Jesus Christ or to blasphemy the Holy Spirit. You dont need to live in order to make these choices, as they have been made before we were born! Wheres the Book of Life when we need it?