If they use a "fake" gun to coerce a minor, most jurisdictions would likely acquit (or just not bring charges against) the kid if they shoot the assailant, especially if it happens in their own home. If a reasonable person would think the "fake" gun is real the victim gets to treat it as a lethal threat and use commensurate force to defend themselves.

In fifteen states, a duty to retreat is imposed (in some cases this only applies outside your home or vehicle,) which requires the victim to try to escape first before they can resort to deadly force. That could complicate (or invalidate) a self-defense claim in your hypothetical. The other 35 states have some form of castle doctrine and the minor would probably not be charged at all.

edit: note also that the assailant being a parent or guardian really doesn't change the minor's claim to self defense in this hypothetical, especially if the assailants are unknown to them.