California Code, Welfare and Institutions Code - WIC § 280

Except where waived by the probation officer, judge, or referee and the minor, the probation officer shall be present in court to represent the interests of each person who is the subject of a petition to declare that person to be a ward or dependent child upon all hearings or rehearings of his or her case, and shall furnish to the court such information and assistance as the court may require.  If so ordered, the probation officer shall take charge of that person before and after any hearing or rehearing.

It shall be the duty of the probation officer to prepare for every hearing on the disposition of a case as provided by Section 356, 358, 358.1, 361.5, 364, 366, 366.2, or 366.21 as is appropriate for the specific hearing, or, for a hearing as provided by Section 702, a social study of the minor, containing such matters as may be relevant to a proper disposition of the case.  The social study shall include a recommendation for the disposition of the case.