When children come to CPS's attention, the Social Worker is solely responsible for determining whether to start Juvenile Dependency proceedings. 

Welfare & Institution Code §328 states: 

“If the social worker has cause to believe that there was or is within the county, or residing in the county, a person described in Section 300, the social worker shall immediately make any investigation the social worker deems necessary to determine whether child welfare services should be offered to the family and whether proceedings in the juvenile court should be commenced. If the social worker determines that it is appropriate to offer child welfare services to the family, the social worker shall make a referral to these services.” 

Moreover, the California State CPS Policies under Division 31 Regulations, 31-125, outline a Social Worker or CPS's INVESTIGATION REQUIREMENTS. 

“The social worker initially investigating a referral shall determine the potential for or the existence  of any conditions(s) which places the child, or any other child in the family or household, at risk and  in need of services and which would cause the child to be a person described by Welfare and  Institutions Code Sections 300(a) through (j). 

The social worker shall not determine the child to be at risk and in need of services, or to be a  person described by Welfare and Institutions Code Section 300(a) through (j) based solely on  the existence of any of the following conditions described in Welfare and Institutions Code  Sections 300(a) through (c): 

1. "...reasonable and age-appropriate spanking to the buttocks where there is no evidence  of serious physical injury," 

2. "...lack of an emergency shelter for the family," or 

3. "...the willful failure of the parent or guardian to provide adequate mental health  treatment...based on a sincerely held religious belief."