I think it means the "actor" is the one committing the offense? /MOO I don't know for sure
From:
http://docs.legis.wisconsin.gov/statutes/statutes/948/10
948.10  Exposing genitals or pubic area.
(1) Whoever, for purposes of sexual arousal or sexual gratification, causes a child to expose genitals or pubic area or exposes genitals or pubic area to a child is guilty of the following:
(a) Except as provided in par. (b), a Class I felony.
(b) A Class A misdemeanor if any of the following applies:
1. The actor is a child when the violation occurs.
2. At the time of the violation, the actor had not attained the age of 19 years and was not more than 4 years older than the child.