First name is a locally unique identifier for some period of time (usually a lifetime, otherwise for at least a decade) until a child will change it for any reason (e.g. immigration, conversion to a different religion, gender re-assignment, a recommendation from a personal psychic, or simply not liking it).
But yes, it's not a true semantic key since the first name usually doesn't come from an immutable properties of the child, but the last name can be a semantic key, derived from the properties of the family (e.g. geographic origin, profession, hair color, look, etc.)
