Were they actually cleared? How could they be when cops couldn't account for their movements that day and no witnesses to the walk? There's no evidence they were involved but that doesn't mean they weren't. When they dug up their garden they found "an area of interest" but no body.
A really unique case and very strange one.
Perhaps this post has eluded you:
The parents were cleared of the charges after a 17 week investigation.
Who are Steven Clark's parents Doris and Charles?