It shall be unlawful for any person under the age of twenty-one (21) years to enter upon premises where alcoholic liquors, spirits, beer or wine are sold by the holder of a license unless accompanied by a parent or legal guardian. No holder of a Class “A” or a Class “B” license, nor any officer, associate, member, representative, agent or employee of such licensee shall permit any person under the age of twenty-one (21) years not accompanied by a parent or legal guardian to enter the licensed premises. For the purpose of preventing the violation of this Section, any holder of a Class “A” or a Class “B” license, or his agent or employee, may refuse to permit entry onto the licensed premises of any person under the age of twenty-one (21) years who is unable to produce adequate written evidence of the fact that the person accompanying such person under the age of twenty-one (21) years is that person's parent or legal guardian. (235 ILCS 5/4-1)