Is the 8 year old mature enough to understand sex? Truly understand the risks it can have? Does she know about consent and how it works, that she can revoke consent at any time? Is the other party actually responsible and follows consent?

This basic series of questions determines the answer, and if any are "no" then no, she shouldn't be in a sexual relationship. 8 yr olds are usually too young for all that imo, but when they actually start seeking out sexual experiances because of hormones (like age 11-13) they are usually mature enough to learn that stuff, and if not then just let them be, and let them check in after awhile if they want to