Assalam-o-Alaikam. I am not a scholar but this world is not created to be just. A just world will be the next world. This world is created to test people if they follow the message of Allah. Suffering of a person, child or animal can only be recognized by the one who is suffering. Only that person suffering can feel. But, one thing is for sure, others suffering is a test for those who are observing them.
When I was around 5 year of age, I went through many pains, diseases and sufferings, but I never felt that way. I am sure my parents went through more mental anguish than the pain I went through.