California Code, Welfare and Institutions Code - WIC § 1500

A peace officer of any city or county shall prevent the entry from California into the Republic of Mexico at the border by any resident of this state under the age of 18 years who is unaccompanied by a parent or guardian or who does not have written consent for such entry from a parent or guardian or who does not have a passport. The authority of the peace officer under this part shall be only to prevent entry and not otherwise to detain. Nothing in this part shall be construed to limit the authority of a peace officer under any other law of this state.