Tirmidhi

1:
In this, there is evidence for al-Shafi‘i, Ahmad, Malik, and the majority of scholars who hold the view that the pilgrimage (Hajj) of a minor child is valid, and he will be rewarded for it, although the obligation (fard) will not be lifted from him. When he attains maturity and has the ability, he will have to perform Hajj again, and the Hajj performed by the minor will be considered a voluntary (nafl) Hajj.
Imam Abu Hanifah says that the Hajj of a minor child is not established; he only performs it for practice and training, and he will not receive any reward for it.
This hadith is an argument against their view.

Benefits and Issues:
➊ The pilgrimage (Hajj) of a minor child is valid, but it is considered a voluntary (nafl) Hajj. After reaching maturity, if the person has the ability, performing Hajj again becomes obligatory.

➋ The parents or guardians of the child receive reward because they train the child for Hajj and bear the hardship of taking the child along. They also perform acts such as stoning (rami) and sacrifice (qurbani) on the child’s behalf. Similarly, during the circumambulation (tawaf) and the walking between Safa and Marwah (sa’i), sometimes they carry the child and perform tawaf and sa’i for him. However, in this case, the tawaf and sa’i are counted on behalf of the child; the one carrying the child must perform their own tawaf and sa’i separately.