Praise be to Allah, and peace and blessings be upon the Messenger of Allah. To proceed:
Among the foundations of faith is that Allah تعالى is the Most Just of judges and the Most Merciful of those who show mercy, and He does not wrong anyone. He سبحانه says:
“And your Lord does not wrong anyone.”
And He says:
“Indeed, Allah does not wrong even the weight of an atom.”
Part of His justice is that He does not punish anyone before the proof has been established against him.
The scholars of Ahl al-Sunnah agree that Muslim children who die before reaching puberty are in Paradise, because a child has not yet reached the age of religious responsibility.
First: Because the child is not legally responsible.
The Prophet ﷺ said:
“The pen has been lifted from three: from the sleeper until he awakens, and from the child until he reaches maturity...”
Narrated by Abu Dawud, al-Nasa'i, and Ibn Majah; authenticated by al-Albani.
Thus, a child who dies before puberty was not legally responsible for the rulings of Islamic law. Therefore, it is part of Allah's perfect justice that He does not hold the child accountable for deeds concerning which he had no legal responsibility.
Second: The child is born upon the natural disposition (fitrah).
The Prophet ﷺ said:
“Every newborn is born upon the fitrah, then his parents make him a Jew, a Christian, or a Magian.”
Narrated by al-Bukhari.
If the child dies before puberty, he is regarded as one who has not reached the age of responsibility. Therefore, it is fitting that he enters Paradise without reckoning.
Third: It is a mercy and honor for his parents.
Losing a young child is a tremendous calamity. Among Allah's mercy is that He grants the child honor in the Hereafter, thereby bringing comfort to the hearts of his parents.
These are some of the wisdoms mentioned by the scholars. They do not necessarily constitute all the wisdoms, for there may be other purposes and wisdoms behind this that we do not know.
The distinction between a child and an adult is therefore clear: an adult is obligated to believe and obey and is held accountable for his deeds, whereas a child before puberty is not legally responsible. It is part of Allah's justice and mercy that He does not hold him accountable for something for which he was not responsible.
And Allah تعالى knows best.