Will a girl be sinful if she makes her fiancé swear that no one has touched her body, while she was subjected to sexual assault in her childhood? And is she considered sinful for what happened to her when she was eight years old?

You are not obligated to tell him what happened previously, and he has no right to ask about it. If he asks and you need to answer him, and he makes you swear an oath, then use tawriya (ambiguous wording) in the oath. There is no sin upon you for the assault that occurred.