Is it forbidden to have sexual relations at the age of eight, and is expiation required for it?

A child who has not reached puberty is not accountable for sins and is not obligated by their actions, based on the Prophet's (peace be upon him) saying: "The pen has been lifted from three: from the sleeper until he awakens, from the child until he reaches puberty, and from the insane person until he regains his sanity."

However, it is incumbent upon the guardian of the child to provide good upbringing, keep them away from sins, and encourage them towards good deeds. If the guardian falls short in this, they will be held accountable before Allah, as Allah Almighty says: "O you who have believed, protect yourselves and your families from a Fire" (Quran 66:6), and as the Prophet (peace be upon him) said: "Indeed, all of you are guardians, and all of you are responsible for your charges."