I don't think they could face any charges for refusing to talk. Everyone has a right to remain silent, whether they're charged with a crime or not. Even if they're charged and taken to trial, it is their absolute right to take the 5th amendment on the stand and it is not to be held against them.
Imo, if the parents continue to refuse LE's requests for separate interviews and no big breaks in the case emerge, the best chance LE has to hear about goings on in the house in relation to the children (the night of the 3rd and all other nights) will come in civil/family court if Ms. Raim follows through with a custody suit. If Jeremy and/or Debbi refuse to answer questions about that night in the custody suit, it is my understanding that it will be a huge strike against them in retaining custody. The judge needs that information to determine the best interest of the chid. The testimony from the custody suit could then be used by LE in a possible criminal case against the parents. If Jeremy and Debbi take the 5th in civil/family court, Jeremy could end up losing custody of his son to the mother (or to someone else if the mother is deemed unfit), like Terri Horman did. Terri couldn't fight for custody of her daughter because she would have been required to answer questions and defend herself regarding her activities and behavior with the children and at the home, testimony which LE then could have used if they decided to charge her in relation to Kyron's disappearance.
ETA: It might well boil down to Jeremy and Debbi having to decide between answering some of the same questions LE wants answered in civil/family court OR continuing to refuse answering those questions and losing custody of the 8 year old boy.
JMO...