What is the wisdom behind the brother who died in childhood not attaining the same rank as his brother in Paradise, and why was the brother who entered Hell in childhood not taken [i.e., his soul not taken] to enter Paradise?

What the questioner mentioned is an objection raised by Sheikh Abu al-Hasan (definition) al-Ash'ari against al-Jubba'i the Mu'tazili. The Mu'tazilah used to say that it is obligatory for Allah to do what is most beneficial for His servants. The Sunnis countered the Mu'tazilah with various refutations of their doctrine, among them what Imam Abu al-Hasan al-Ash'ari posed to al-Jubba'i concerning the three brothers (one of whom died young, and the other two were kept alive, one choosing faith and the other choosing disbelief). Then al-Ash'ari asked: If the disbeliever says on the Day of Resurrection, "Why did You not cause me to die young so that I would not disobey and thus not enter Hellfire?", what would the Lord say? Al-Jubba'i was dumbfounded. And Allah, Exalted be He, creates what He wills and chooses, He is not questioned concerning what He does, while they will be questioned.