Musannaf Ibn Abi Shayba 13:166

Sufyan ibn 'Uyaynah narrated to us, from al-Zuhri, from Sa'id, who said: That was complained about to Uthman, that a child, when he is small, does not possess [legal capacity to take possession]. So he held the view that if his father gifts [something] to him and calls witnesses [to the transaction], it is valid.