According to the view that is deemed to be the most correct, if a child performs Hajj, then his/her Hajj is valid and he/she will be rewarded for it, in sha’ Allah. However, this Hajj will not count as the obligatory Hajj. The child must make Hajj again (if he/she is financially and physically able) after reaching puberty. This view is backed by Prophetic Hadiths as well as the consensus of Muslim scholars. However, the child’s father or guardian should inform him about the acts that are prohibited during ihram and should help the child perform the rituals.
Sheikh Muhammad Saleh Al-Munajjid, states the following:
Muslim scholars agree that Hajj and `Umrah are not obligatory upon those who have not reached the age of puberty, because the Pen is lifted from children, as the Prophet (peace and blessings be upon him) said: “The Pen is lifted from three (i.e., their deeds are not recorded): a child until he reaches puberty; an insane man until he comes to his senses; and one who is asleep until he wakes up.” (Reported by Abu Dawud, 4403; Ibn Majah, 2041)
However, if a child performs Hajj, then his Hajj is valid and he will be rewarded for it according to the prevalent view which is maintained by the majority of scholars, and it was narrated that there is consensus on this point.
Ibn `Abbas, (may Allah be pleased with him) reported that the Prophet (peace and blessings be upon him) met some riders in al-Rawha’ and said, “Who are these people?” They said, “Muslims.” They said, “Who are you?” He said, “The Messenger of Allah.” A woman lifted up a child and said, “Is there Hajj for him?” He said, ‘Yes, and you shall have a reward.” (Reported by Muslim, 1336.)
Having said that, we would like to stress that if a child has reached the age of discernment but has not yet reached puberty and his father or guardian wants to take him for Hajj, he should tell the child to put on ihram garments and the child should do all the rituals of Hajj for himself, starting with entering ihram from the miqat, until the end of the rituals of Hajj, and his father or guardian should stone the jamarat on his behalf if he cannot do that by himself.
The guardian of the child should tell him to avoid things that are prohibited during the ihram. However, if the child is below the age of discernment, then his father or guardian should make the intention of entering ihram for Hajj or `Umrah on the child’s behalf (while making the intention of entering ihram on his own behalf as well). He should then perform tawaf or circumambulation around the Ka`bah), and sa`i or running between the two hills of Safa and Marwa with the child and accompany the child in performing the other rituals.”