Police somehow established that Karlie was killed in Belanglo Forest on 14/15 December 2008. I believe to narrow her death to two dates or spanning the two dates, they must have had irrefutable proof from someone. It is likely that they also have established that Kandalyce was gone by end of December 2008 from a similar source. It could be neighbours relative to where they were staying, a shopkeeper, doctor, appt, child carer or any number of other sources.
Or, it could be something Holdrum himself said, .. somehow, I cant see him coughing up the story that he murdered a tiny child in cold blood, not to a couple of toughish NSW coppers.. the image is all, if he did it, it will have to be wrung out of him, because it doesn't sit well with other prisoners, when one gets shunted off to Goulburn .. I can see him shoving the story off on to someone else, at this point in time, either by claim or indication.
Or a photo somewhere. Some one has a pic of Khandalyce, without Karlie, that can be verified absolutely at a certain date. After Karlie's death. And the NSW D's are pretty certain of her date of death.
Or ... the coppers know it all and are playing a long game as is so often done in these sorts of enterprises... .. that sounds about right.