I think body language experts are usually full of you-know-what, especially in cases like this. There is simply no template for how a parent should act when a child goes missing.
One thing I do think is telling in watching these kinds of interviews, is crying-Is the parent crying? Is the display of emotion genuine, or are they scrunching up their face and making crying noises, but no tears are coming out? And, IMO most important-WHEN are they crying? At what point in retelling the story of their child's disappearance do they show the most emotion?
Both mom and dad cried "appropriately", for lack of a better word. Jessica's mother, for example, held it together fairly well until she started to describe that "pit in the stomach" feeling when she realized her child was taken. This is exactly the point I would expect a parent to break down if their child has gone missing and they are not involved. Because that is the moment it becomes every parent's worst nightmare.
AND, it is very possible that the mother or father has been heavily medicated, as you can imagine they would need to be. Sedatives, sleeping pills and anti depressants.