If the murdered person is under what is known as the age of accountability, he will be safe with the Lord in heaven. This is not an exact age but a recognition that babies and young children have not yet reached the ability to know right from wrong and are therefore not yet personally responsible for their actions. Only God knows exactly when someone passes from innocence to responsibility, but there is certainly a time when this happens. When King David lost a baby boy, he recognized that he would see him again. He said, "I shall go to him, but he shall not return to me" (2 Samuel 12:23). Isaiah 7:16 teaches that there is a time when a child "shall know to refuse the evil, and choose the good." This is probably a description of the age of accountability.
If, however, the young person was past the age of accountability and was murdered, he would be responsible for his own sins before God. In this case, if he had not been saved, he would be lost. This is why it is so important to reach children with the gospel while they are still young. Though this may seem unfair, we must remember that there are many people in various parts of the world who live to old age without hearing a clear presentation of the gospel. We are to go and tell them of Jesus Christ. Paul rebuked the Corinthians, "for some have not the knowledge of God: I speak this to your shame" (1 Corinthians 15:34).
If the murderer truly comes to the Lord Jesus, repents of his sin, and trusts in Him as Lord and Saviour, then he will indeed go to heaven. The Apostle Paul was a persecutor of Christians and considered himself the chief of sinners; yet he got saved (1 Timothy 1:13-15). Certainly, the grace of God is sufficient to save even a murderer.
I do not know whether the instance you give in your question is a real situation or an imagined one. However, it has probably happened at some time in the past. It definitely seems unfair. But I believe concern over such a situation comes from a lack of understanding of the total grace of God that is required for the salvation of any person. No one deserves heaven. We are all guilty and we all have the potential to commit horrible sins. Ultimately, problems like this need to be left in the hands of the righteous God and Judge of all.