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, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
There is no doubt that a child (who has reached the age of distinction) is rewarded for the good deeds he offered before the age of puberty, based on the Hadeeth narrated on the authority of Kurayb when he said: "A woman raised a child and said: 'O Messenger of Allaah. Is this child rewarded for Hajj?' The Prophet, sallallaahu ‘alayhi wa sallam, replied: 'Yes, and you (too) are rewarded.'" [Muslim]
Furthermore, it is narrated on the authority of ‘Abdullaah ibn ‘Amr that the Prophet, sallallaahu ‘alayhi wa sallam said, "Command your children to perform prayer when they are seven years old and beat them for not performing it when they are ten years old." [Ahmad and Abu Daawood: Saheeh]
Allaah Knows best.