I think the parents were guilty, but not of murder or even cover ups. I think they had a somewhat cold appearing demeanor, were superficial and overly focused on perfect appearances, even with their children. I'm not a fan of the pageant thing at all.
But I think these were the things that convicted them in the public eye. I really don't think any of the immediate family did it. I think it very possible that some creepy person they knew, possibly connected with the dad's work, seeing a picture of the overly beatified, overly matured little girl, and concocting a plan to steal all the wonderful things this family appeared to have. I can see the Ramsey's attitude rubbing some crazy, bitter, hateful person the wrong way enough to plant the seeds of a horrible plan.
Of course none of us know, because the evidence was so completely messed up. But this type of scenario makes the most sense in my mind. Incredibly sad story - whatever the truth is.
E.