Short Answer

Children are rewarded for the good deeds they do even before they have reached the age of maturity. Children are not accountable for their bad deeds until they reach the age of maturity. Therefore, the bad things they do are not considered sins.

What the Qur’an and Sunnah Say

It was reported that: “A woman lifted up a child and said, ‘Messenger of Allāh, will his Hajj be counted?’ He said, ‘Yes, and you will be rewarded.'” [Al-Bukhārī #1336]

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Shaykh Mustafa Umar