You are making a correlation between a visitation issue and a missing person investigation where factually one has not been determined to exist as far as we know. There's basically no scenario that one can imagine where the children have not been interviewed extensively by the police already and possibly on more then one occasion since they have also been in their grandparents' custody at some point after MP disappearance, there is also no factual indication that DS has prevented the children from being interviewed by the police that I'm aware of, and if any of it is not enough, I believe the police in this particular case would be able to interview the children without parental consent pursuant a court order. JMO
I may be mistaken and I'm sure someone can correct me if I am, but I believe that the children were interviewed once shortly after Michelle disappeared and only for a short time. Now that may have been when DFS was involved and removed the children from DSJr's custody for a brief period just prior to the emergency court hearing.
MOO