Even if LE didn't know where Chad and Lori were in late Nov (though they somehow knew the couple had "left the state"), I think they know now - but are waiting until they have bulletproof evidence of a major felony/crime to charge one or both of them (not to restart the whole "not knowing or divulging where your kids are" is a crime debate). I think LE wants to have as much leverage as possible (via the charges) in order to squeeze one or both of them into divulging the location of the children (which I think rightly has been everyone's main focus from the start).
The FBI is pretty good at finding people (license plate cams, vehicle GPS, cell phone triangulation, etc). Among other places, Chad & Lori could be in AZ, Vegas, UT, HI, or even in Missouri - specifically in the Gallatin area, or in between Jameson, Hwy 13, Gallatin, and the Grand River. Maybe that is a possible location to look for the children as well