Romans 9:14-18
14 What shall we say then? There is no injustice with God, is there? May it never be! 15 For He says to Moses, "I will have mercy on whom I have mercy, and I will have compassion on whom I have compassion." 16 So then it does not depend on the man who wills or the man who runs, but on God who has mercy. 17 For the Scripture says to Pharaoh, "For this very purpose I raised you up, to demonstrate My power in you, and that My name might be proclaimed throughout the whole earth." 18 So then He has mercy on whom He desires, and He hardens whom He desires. NAS
Children do not have the ability to determine right from wrong or good from evil. It is fully within God's right to have mercy upon children who die before they have the mental capacity to determine the difference between good and evil; to believe He will judge children is to view God as a God wrath and a God of injustice.