Will a person be rewarded for the good deeds they perform before reaching puberty?

Yes, a child is rewarded for good deeds, and misdeeds are not recorded against him. His deliberate actions are treated as mistakes. This is the view of the majority of scholars, and some have even cited a consensus on this matter. This is supported by the hadith of the woman who lifted a child and asked the Prophet, peace and blessings be upon him, "Is there Hajj for this one?" He replied, "Yes, and you will have a reward."