A licensee of a premises hereunder and the driver of any motor vehicle in the city, in which any person under the age of 21 years possesses or consumes any alcoholic liquor otherwise than as authorized herein, shall be deemed to be in violation of this chapter. It shall be an affirmative defense in any prosecution of the licensee or driver for violation of this subchapter that the licensee or driver did not know that a violation hereof would occur or was occurring, did not have reasonable grounds to believe a violation would occur or was occurring, and could not have reasonably done anything to prevent the violation from occurring.
Penalty, see § 111.999