All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
If a child commits sin before his puberty, then he is not held accountable for it since he is still below the age of being held accountable for his acts. The Prophet said: "The pen is lifted (stopped from writing the deeds) from three kinds of people: a sleeping person until he wakes up, a mentally-handicapped or insane person till he can reason, and a child till he reaches the age of puberty."
However, if the sin is related to the right of others such as stealing from someone, then it should be paid back to the rightful person if he is available, and if he is dead, it should be paid to his inheritors if they are known, otherwise the object should be given as charity.
For more benefit, please refer to Fatwa 81223.
Allaah Knows best.