Vrijwillige rechtspraak means voluntary justice.
You may come across the term in finding aids of court records. The series of vrijwillige rechtspraak are cases where the court is involved, but nobody is charging or sueing anyone. Examples of records you may find in the series of voluntary records are prenuptial agreements, guardianship appointments, last wills, or estate divisions. In regions that had notaries, you may encounter those types of records in the archives of the notaries instead.
Would that then include adoption?