The Hajj of a minor child is valid, but it does not count as the obligatory Hajj. Rather when the person reached adolescence he has to do Hajj again when he is able to. This is because of the hadeeth Ibn ‘Abbaas Radhiallahu Anhu, who said: The Messenger of Allaah Sallallahu Alaihe Wa-sallam said: “Any child who does Hajj then reaches adolescence has to do another Hajj.” [Narrated by Ibn Abi Shaybah and classed as Saheeh by Ibn Hajar in al-Talkhees, 2/220, and by al-Albaani in al-Irwa’, 4/159.] It was narrated from Ibn ‘Abbaas Radhiallahu Anhu that the Prophet Sallallahu Alaihe Wa-sallam said: “Any child who does Hajj then reaches puberty, he must do Hajj again.” [Narrated by al-Shaafa’i in his Musnad; classed as Saheeh by al-Albaani in Irwa’ al-Ghaleel (686).