The affidavit for a search warrant that OCSO completed on Dec. 11, 2008 contained reference to the remains being found in a black bag, a blanket with the remains, and duct tape.
As I understand it, the affidavit would normally be presented with the search warrant itself. So if George and Cindy were served with the paperwork that evening then they would be aware of what was found. I recall seeing video of them at the house retrieving a dog, but I don't know if that was before or after the search started.
The Anthony's had dinner with Baez that evening, so if the warrant had been served by dinner time to Cindy, George, Brad, Hoover, or DC then it is possible the information got to Jose at that time. Then, around 8PM that evening, Jose paid a visit to KC. This would be the earliest possible point, IMO, that information about the blanket and bag could be passed on to KC.
Even that is not a guarantee, and it would be worth tracing out the timeline of where the Anthony's were and when on the 11th to see if it is possible they or someone in their camp even saw the warrant or affidavit that evening. If they did not, the next visit with KC occurred on Dec. 14, providing ample time for KC to reveal inside information.