Wisconsin
Tattoo artists and establishments where tattoos are performed must be licensed by the state health department, and establishments must be inspected before a licence is issued (statute 252.23). Alternatively, in areas with a population over 5000 people, the state health department can authorize a local health department to do inspections and licensing (statute 252.245).
No one may tattoo children except doctors doing so in the course of their medical practice: "(2) Subject to sub. (3), any person who tattoos or offers to tattoo a child is subject to a Class D forfeiture.". (3) Subsection (2) does not prohibit a physician from tattooing or offering to tattoo a child in the course of professional practice.
Read more about this topic: Legal Status Of Tattooing In The United States