Cover up? It depends on what they knew, when they knew it and their pattern of dealing with knowledge of uncomfortable facts. Go back to their denial of an obvious 7 month pregnancy at the wedding in June 2005. Of course they knew KC was pregnant.
So, when did they know something was wrong? They certainly knew KC very well. They raised her and she continued to live with them after she became an adult.
They knew about the big fight on June 15, 2008. They knew every detail of their botched "intervention" about the quality of KC's parenting.
When CA denied that KC would ever abuse Caylee, the red flags went up. The opposite was more probably true. The computer searches around KC's 22nd birthday in March for shovels, making chloroform and neck breaking didn't suddenly come out of the blue from a lovely, compassionate, innocent 22 year old.
The sheer volume and vehemence of the denials by CA and to some extent GA are proof in and of themselves that they knew.
GA probably knew first when he tried to get into the trunk of the car for the tire chocks. No doubt, he probably talked to CA. After that, CA picked up the pace in trying to find KC and Caylee. After that came the web posting by CA. It appears CA knew there was a probable homicide as early as the web posting. IF KC brought a dead body into the house and laid it on the bed as some suspect, there was probably a stench in the house by no later than June 19th. CA washed the sheets more than once -- for a child who was not there. The confrontation on July 15th was just the dance this family has danced for years and years -- decades -- when they want to get the truth from KC. It was a predictable script. CA knew she was going to have to call the police as soon as they picked up the car before she ever found KC.