The process varies greatly not only by state, but by jurisdiction within each state. I just today had a conversation with a friend who left CPS in my county and went to another state where they are "years behind" CA in the legal and social arena. So how these things are handled can vary greatly.
The OP is in CA. Out here, generally, the caseworkers have heavy caseloads and they have to close them. For the most part, this requires something after the report. This means an interview, an inspection, something. Being rebuffed at the door will keep the matter open and may keep the social worker looking for other complaints, police contacts or radio calls, or just about anything where he or she can fish for additional information or to contact the children or the parent(s). The best way I have found to prevent ongoing scrutiny over baseless accusations is to show that they ARE baseless. Refusing to cooperate - while perfectly lawful - lends an air of suspicion to the proceeding that might not have otherwise been present as the case worker asks, "Hmmm ... what are they hiding?" And I have found that many of these initial contact caseworkers are the least experienced ones so they may go with the typical gut reaction that says if they are refusing to cooperate they are hiding something (young cops do the same thing).
Certainly, one can refuse to cooperate. No one can guarantee that cooperation will keep them out of trouble. But, failure to cooperate will almost certainly guaranteed continued attention.