How is the view of guaranteed salvation for infants justified? Many Christians hold the view that unborn and born infants and children who die before the age of reason or eternal responsibility are guaranteed access to heaven. For instance, Dr. William Lane Craig has famously stated the following, When justifying the slaughter of the Canaanites:


  Moreover, if we believe, as I do, that God's grace is extended to those who die in infancy or as small children, the death of these children was actually their salvation.


It seems obvious that if a child/infant dies before they are responsible for accepting salvation they should not be condemned to Hell, but that they should be automatically granted salvation also seems equally unjust. I would think that it be more just for God to annihilate the souls of those children.

How, in this view, is God just by automatically saving all infants/children who die before they can be held responsible?

As Matt Gutting pointed out, the question is about how someone holding the view that infants/children who die in infancy are guaranteed salvation can reconcile their view with God's justice.
Firstly, we need to know that God is just and fair.


  God is just: He will pay back trouble to those who trouble you. (2 Thessalonians 1:6, 
  NIV)
  
  But you are stubborn and refuse to turn to God. So you are making
  things even worse for yourselves on that day when he will show how
  angry he is and will judge the world with fairness. (Romans 2:5,
  CEV)


Secondly, we must also believe that just as Heaven is real and eternal, Hell is also real and eternal. Though there are some who believe in Annihilationism, they are minority. 


  "Then they will go away to eternal punishment, but the righteous to
  eternal life." (Matthew 25:46, NIV)


Is the view of guaranteed salvation for infants unjust? Not entirely.

The Catholic Church believes that unbaptized infants go to Heaven. 

The Bible never tells us that innocent children will go to Hell or not. But we know for sure that Jesus said that the Kingdom of God belongs to children.


  Jesus said, "Let the little children come to me, and do not hinder
  them, for the kingdom of heaven belongs to such as these." (Matthew
  19:14, NIV)
  
  And he said: "Truly I tell you, unless you change and become like
  little children, you will never enter the kingdom of heaven. (Matthew
  18:3, NIV)


This seems like a contradiction to the teaching of Saint Paul that all have sinned.


  for all have sinned and fall short of the glory of God (Romans 3:23,
  NIV)
  
  Surely I was sinful at birth, sinful from the time my mother conceived
  me. (Psalm 51:5, NIV)


However, the Bible tells us that we are going to be judged according to what we have done, not what we haven't commit. 


  For God will bring every deed into judgment, including every hidden
  thing, whether it is good or evil. (Ecclesiastes 12:14, NIV)
  
  God "will repay each person according to what they have done."
  (Romans 2:6, NIV)


Some adults might say that they have not done any serious crime but in God's standard all sins are big, for Jesus said,


  "But I tell you that anyone who looks at a woman lustfully has already
  committed adultery with her in his heart." (Matthew 5:28)


Therefore, it is fair and just to assert that children who died young before committing any sin will go to Heaven but adults can go to Heaven only by believing in Jesus Christ.

There are many interesting testimonies around the world - people who claimed to have visited Heaven. Out of them, I find these two very interesting as they deal with one of the toughest questions which Christianity cannot answer objectively. 


In the book Heaven Is for Real, Colton, a pastor's 4 years old son went to Heaven and said that he met a little girl in Heaven who said she was his sister who died in his mother’s “tummy”. 
In his interview video "An insight into heaven"(please search on YouTube), D. G. S. Dhinakaran, the founder of Jesus Calls Ministries also said that he saw infants in Heaven and they are taken care of until they reach 30 years and they become angels. He said that children who died young are not given the saved status and do not enjoy the privileges like the saved, they are simply given a basic entry to Heaven.