Wis. Stat. § 88.10

Failure to appoint a guardian ad litem in a proceeding under this chapter is not jurisdictional, but when the failure is discovered a guardian ad litem shall be appointed and an order served upon the guardian ad litem to show cause why the minor or individual adjudicated incompetent should not be bound by all prior proceedings pertaining to the drainage district. On such a hearing the court shall enter such order or judgment as the facts warrant.