The problem of suffering and the why of tragedy continues to haunt mankind. The book of the Bible that was written down before all others was most likely the book of Job--and its main theme is the purpose of suffering. In the last book of the Bible, the book of Revelation, the slain saints cry out, How long? (Revelation 6:9-10). This cry deals with the same subject. This is a massive subject and I can only give a few thoughts in a short answer like this.
Although I cannot say that God would never use such a tragedy to turn someone to Himself, to declare such a thing as a matter of fact is a trite and cruel answer. First of all, it is as likely to turn someone from God as it is to turn them to God. Second, I have known some very godly parents to suffer similar tragedies. Third, Job suffered the tragic loss of all his children partially as a result of God bragging on his righteousness. And the friends of Job who assumed wrong-doing on his part because of the tragedies were condemned by the Lord and had to get Job to pray for them in order to obtain forgiveness for their sins. No, this is never the answer man should give to others. We simply do not know.
God's answer to Job may give some help. However, it is not the one Job (or I) expected. Simply put, God told Job that God is so exalted above man in wisdom and power that Job could not understand the ways of God. Inherent in this answer is the teaching that man must trust in God for the things he does not understand. Now I realize that this is not an easy thing to do. To tell someone to just trust God is akin to accusing them of doing something wrong that caused the tragedy. However, this is the ultimate goal of the godly mind. We are to learn to trust God without understanding the reasons.
But, does the Lord give anything else to help us get to this? I think He does. For one thing, we must understand that we live in an evil world caused by sin and rebellion against God. The Bible shows throughout that innocent people suffer in such an evil world. The children killed in wars and famine over the millenniums of the past were not being punished for their ungodliness. They were victims of the sin and resultant suffering found throughout the world. Bad things happen because man has turned his back on God. Unfortunately, these bad things also happen to innocent people. It is the result of the wicked state of the world. It does not have to be the particular cause of a particular sin.
A verse that has often helped me in dealing with those going through such a situation is Isaiah 57:1 - "The righteous perisheth, and no man layeth it to heart: and merciful men are taken away, none considering that the righteous is taken away from the evil to come." This verse teaches us that God may in mercy take someone away from an evil that is to come later. God may have seen good in the boy and have also seen some evil he would face in later years. The taking of the boy may have been an act of love and not an act of hatred.
In the Old Testament, one of the sons of the wicked king Jeroboam died as a child. God gave this reason for taking the boy at such a young age: "And all Israel shall mourn for him, and bury him: for he only of Jeroboam shall come to the grave, because in him there is found some good thing toward the LORD God of Israel in the house of Jeroboam." That is, the death of a child could actually be an act of love. Remember, if the child had not reached the age of accountability (or had been saved), he is not worse off. He is in peace with God. Those who are left behind are the ones suffering. In these situations we must understand that we are not mourning for the child, but for ourselves. Jesus said, "Suffer the little children to come unto me, and forbid them not: for of such is the kingdom of God" (Mark 10:14). He is with God. We are the ones left behind.
David, when he lost his infant son in death, ceased to mourn, washed himself, and prepared to eat. When his servants questioned his actions, he told them, "But now he is dead, wherefore should I fast? can I bring him back again? I shall go to him, but he shall not return to me" (2 Samuel 12:23). The boy could not come back to him, but he could go to the child. That also must be given as the hope that this family needs to take hold of. This is God's comfort to them.
Of course, mere words are always too frail to help in such a time as this. Seek God for this family and encourage them to seek God's help and comfort for themselves. God is the only One who can bring good out of such a tragedy as this. He is certainly willing and desires to help them in this time. I know you will do your best to point them to Him.