The statistics are quite clear that the parents are always the most likely culprits when a child dies. Abduction is vanishingly rare - particularly from inside a property. Much, much more likely that her parents killed her.
And actually, their behaviour afterwards is what aroused most suspicion. They refused to co-operate fully with police and failed to answer certain key questions at a time when you'd have thought they'd do anything to get their child back. Hardly 'despair'.