Every employer, during the period of employment of a minor under 16 years of age, shall keep on file at the place of employment an employment certificate issued for such minor. Any employer, upon termination of the employment of such minor, shall immediately return the certificate issued to the issuing officer. An employment certificate shall be valid only for the employer for whom issued, and a new certificate shall not be issued for the employment of a minor under 16 years of age except on the presentation of a new statement of intention to employ. The failure of any employer to produce for inspection such employment certificate for each minor in his establishment shall be prima facie evidence that the minor is employed without a certificate.