(1) Process against a minor who has never been married shall be served:
(a) By serving a parent or guardian of the minor as provided for in s. 48.031 or, when there is a legal guardian appointed for the minor, by serving the guardian as provided for in s. 48.031.
(b) By serving the guardian ad litem or other person, if one is appointed by the court to represent the minor. Service on the guardian ad litem is unnecessary when he or she appears voluntarily or when the court orders the appearance without service of process on him or her.
(2) In all cases heretofore adjudicated in which process was served on a minor as prescribed by any law heretofore existing, the service was lawfully made, and no proceeding shall be declared irregular or illegal if a guardian ad litem appeared for the minor.