I can only shake my head at any suggestion that the parents are guilty. This would require them to pull off the crime of the century, within an impossible timeframe, including the disposal of the body in some way that cannot be detected by modern forensics.
All of this while surrounded by hundreds of potential witnesses in a very public place!
Then they co-operated with the police and urged them to pursue the investigation for well over a decade, spending unprecedented quantities of taxpayers' money in the process. Because that's what you do when you're guilty and want to conceal your crime, apparently!