It can be different in every court and with every paid supervisor this is paid by either one or both of the parties.  Usually, the supervisor will monitor the visit and will write visitation notes for the positives and negatives, if any, that have occurred during the visits.  You can try and either have those notes submitted to the court as an exhibit which typically the courts will look at that but it depends on the circumstances.  You can also subpoena the worker with the notes for trial or for an evidentiary hearing.  It can be important information for the court where the idea of having an unbiased person watching what occurs during the visitation.