Wis. Stat. § 948.081: Patronizing a child.

An actor who enters or remains in any place of prostitution with intent to have nonmarital sexual intercourse or to commit an act of sexual gratification, in public or in private, involving the sex organ of one person and the mouth or anus of another, masturbation, or sexual contact with a person is guilty of a Class G felony if the person is a child. In a prosecution under this section, it need not be proven that the actor knew the age of the person and it is not a defense that the actor reasonably believed that the person was not a child.