It's called abetting and accomplice liability, Penal code 31. Here's more about Pleading the 5th.
Can Any Witness Plead the Fifth?
At a criminal trial, it is not only the defendant who enjoys the Fifth Amendment right not to testify. Witnesses who are called to the witness stand can refuse to answer certain questions if answering would implicate them in any type of criminal activity (not limited to the case being tried). Witnesses (as well as defendants) in organized crime trials often plead the Fifth, for instance.
But unlike defendants, witnesses who assert this right may do so selectively and do not waive their rights the moment they begin answering questions. Also, unlike defendants, witnesses may be forced by law to testify (typically by subpoena).
http://criminal.findlaw.com/criminal-rights/fifth-amendment-right-against-self-incrimination.html
So yes, they can plead the 5th, but they can be still be held in contempt in this situation given that the proceeding is to gather evidence, not as testimony and the defendant in this case is TC for the crime of kidnapping. If the family should succeed in this issue and be granted the ability to depose the witnesses for the purpose of gathering evidence that could lead to the safe return of this child, the witnesses can plead the 5th and be held in contempt of court. In order for the 5th amendment to be used in any case it must meet two criteria and pleading the 5th in this case just doesn't meet the criteria. Besides, they can just offer immunity to anyone who feels they could be convicted of another crime based on the testimony they give. Pleading the 5th also requires a judge to determine whether to allow the privilege of the 5th, and witnesses are held to a different standard than defendants. There has to be a reasonable cause for criminal liability if the witness answers a specific question, and they have to make that assertion on every question answered. It's a gamble to do it because failure to answer questions results in contempt of court. In this case, if we rely on what we know about one of the witnesses, she dropped ET off and went to work. Unless she also kidnapped ET there's not a whole lot she claim would incriminate her. We can at least agree that these witnesses should come forward of their own volition, even if only on an ethical level.
Please correct me if i'm wrong, but this is a civil complaint, not a criminal complaint (action).